Figure 2.1(A) shows raw fruit. The crisp, sharp-tasting fruit becomes soft and sweet when it is cooked. Figure 2.1(B) shows a chemist accelerating the tranformation of ethanol into ethanoic acid, by adding potassium
dichromate and sulfuric acid.
What do these reactions have in common? They are both examples of organic reactions. In this section, you will take a quick look at the main types of organic reactions. You will concentrate on simply recognizing these types of organic reactions. In the next section, you will examine the reactions of specific functional groups and learn how to predict the products of organic reactions.

Addition, Substitution, and Elimination Reactions

Addition reactions, substitution reactions, and elimination reactions are the three main types of organic reactions. Most organic reactions can be classified as one of these three types.

Addition Reactions

In an addition reaction, atoms are added to a double or triple bond. One bond of the multiple bond breaks so that two new bonds can form. To recognize an addition reaction, remember that two compounds usually react to form one major product. (Sometimes two isomers are formed.)
The product has more atoms bonded to carbon atoms than the organic reactant did. A general example of addition to an alkene is given below.

Addition reactions are common for alkenes and alkynes. Addition reactions can also occur at a CO bond. Some examples of addition reactions are shown on the next page.

Substitution Reactions

In a substitution reaction, a hydrogen atom or a functional group is replaced by a different functional group. To help you recognize this type of reaction, remember that two compounds usually react to form two different products. The organic reactant(s) and the organic product(s) have the same number of atoms bonded to carbon.

Alcohols, alkyl halides, and aromatic compounds commonly undergo substitution reactions, as shown in these examples.

Elimination Reactions

In an elimination reaction, atoms are removed from a molecule to form a double bond. This type of reaction is the reverse of an addition reaction. One reactant usually breaks up to give two products. The organic product
typically has fewer atoms bonded to carbon atoms than the organic reactant did.

Alcohols often undergo elimination reactions when they are heated in the presence of strong acids, such as sulfuric acid, H2SO4, which acts as a catalyst. (See the first example below.) Alkyl halides also undergo elimination reactions to produce alkenes. (See the second example.)

Oxidation and Reduction

An important type of organic reaction occurs when there is a change in the number of hydrogen or oxygen atoms that are bonded to carbon. In Unit 5, you will take a close look at oxidation-reduction reactions in terms of the transfer of electrons. As you will learn, oxidation and reduction always occur together. One reactant is oxidized while the other reactant is reduced. In this unit, however, you will focus on the organic reactant only. Therefore, you will deal with oxidation and reduction separately, as they apply to organic compounds. In organic chemistry, oxidation and reduction are defined by the changes of the bonds to carbon atoms in the organic reactant.

Oxidation

In organic chemistry, oxidation is defined as a reaction in which a carbon atom forms more bonds to oxygen, O, or less bonds to hydrogen, H. An oxidation that involves the formation of double CO bonds may also be classified as an elimination reaction. For example, alcohols can be oxidized to produce aldehydes and ketones. Oxidation occurs when an organic compound reacts with an oxidizing agent. Common oxidizing agents include acidified potassium
permanganate, KMnO4, acidified potassium dichromate, K2Cr2O7, and ozone, O3. The symbol [O] is used to symbolize an oxidizing agent, as shown below. Note that equations for the oxidation of organic compounds are often left unbalanced. The purpose of the equation is to show the changes in the organic reactant only.

To identify an oxidation, count and compare the number of C-H and C-O bonds in both the reactant and product. Try it for the following example.

Reduction

In organic chemistry, reduction is defined as a reaction in which a carbon atom forms fewer bonds to oxygen, O, or more bonds to hydrogen, H. Often, a CO bond or CC bond is reduced to a single bond by reduction. A reduction that transforms double CC or CO bonds to single bonds may also be classified as an addition reaction. Aldehydes,
ketones, and carboxylic acids can be reduced to become alcohols. Alkenes and alkynes can be reduced by the addition of H2 to become alkanes. Reduction occurs when an organic compound reacts with a reducing agent. Common reducing agents are lithium aluminum hydride, LiAlH4, and hydrogen gas over a platinum catalyst, H2/Pt. The symbol [H] is used to symbolize a reducing agent. As is the case for oxidation, equations showing the reduction of organic compounds are often left unbalanced.

To identify a reduction, count and compare the number of C-H and C-O bonds in both the reactant and the product. Try it for the following example.

Other Important Organic Reactions

In this chapter, you will also encounter the following classes of organic reactions: condensation reactions and hydrolysis reactions. Condensation and hydrolysis reactions are both types of substitution reactions.

Condensation Reactions

In a condensation reaction, two organic molecules combine to form a single organic molecule. A small molecule, usually water, is produced during the reaction. For example, a carboxylic acid and an alcohol can condense to form an ester.

Hydrolysis Reactions

In a hydrolysis reaction, water adds to a bond, splitting it in two. This reaction is the reverse of a condensation reaction. For example, water can add to an ester or amide bond. A carboxylic acid and an alcohol are produced if an ester bond is hydrolyzed, as shown in the example below. A carboxylic acid and an amine are produced if an amide bond is hydrolyzed.

The following Sample Problems show how to identify different types of organic reactions.

Some of the most interesting and useful organic compounds belong to families you are about to encounter. For example, the sweet taste ofvanilla and the spicy scent of cinnamon have something in common: a carbonyl group. A carbonyl group is composed of a carbon atom doublebonded to an oxygen atom. In this section, you will study the structures and properties of organic compounds that have the C═O group.

Aldehydes and Ketones

Aldehydes and ketones both have the carbonyl functional group. An aldehyde is an organic compound that has a double-bonded oxygen on the last carbon of a carbon chain. The functional group for an aldehyde is

The general formula for an aldehyde is R-CHO, where R is any alkylgroup. Figure 1.18 shows the first two aldehydes.

When the carbonyl group occurs within a hydrocarbon chain, the compound is a ketone. A ketone is an organic compound that has a double-bonded oxygen on any carbon within the carbon chain. The
functional group of a ketone is

The general formula for a ketone is RCOR′, where R and R′ are alkyl groups. Figure 1.19 shows the simplest ketone, propanone.

Like the other organic compounds you have encountered, the names of aldehydes and ketones are based on the names of the parent alkanes. To name an aldehyde, follow the steps below.

How to Name an Aldehyde

Step 1 Name the parent alkane. Always give the carbon atom of the carbonyl group the position number 1.
Step 2 Replace the -e at the end of the name of the parent alkane with -al. The carbonyl group is always given position number 1. Therefore, you do not need to include a position number for it.
To name a ketone, follow the steps on the next page. The Sample Problem that follows gives examples for naming both aldehydes and ketones.

How to Name a Keton

Step 1 Name the parent alkane. Remember that the main chain must contain the CO group.
Step 2 If there is one ketone group, replace the -e at the end of the name of the parent alkane with -one. If there is more than one ketone group, keep the -e suffix and add a suffix such as -dione or -trione.
Step 3 For carbon chains that have more than four carbons, a position number is needed for the carbonyl group. Number the carbon chain so that the carbonyl group has the lowest possible number.

Additional Characteristics of Aldehydes and Ketones

Carboxylic Acids

You are already familiar with one carboxylic acid. In fact, you may sprinkle it over your French fries or your salad, as shown in Figure 1.22. Vinegar is a 5% solution of acetic acid in water. The IUPAC name for
acetic acid is ethanoic acid, CH³COOH.

A carboxylic acid is an organic compound with the following functional group:

This -COOH group is called the carboxyl group. The general formula for a carboxylic acid is RCOOH. Figure 1.23 shows some common carboxylic acids.

To name a simple carboxylic acid, follow the steps below. Figure 1.24 gives some examples of carboxylic acid names.

How to Name a Carboxylic Acid

Step 1 Name the parent alkane.
Step 2 Replace the -e at the end of the name of the parent alkane with
-oic acid.
Step 3 The carbon atom of the carboxyl group is always given position number 1. Name and number the branches that are attached to the compound.

Table 1.12 lists some of the physical properties of carboxylic acids. Notice that carboxylic acids have even stronger hydrogen bonding than alcohols

Additional Characteristics of Carboxylic Acids

• Carboxylic acids often have unpleasant odours. For example, butanoicacid has the odour of stale sweat.
• The -OH group in a carboxylic acid does not behave like the basic hydroxide ion, OH-. Oxygen has a high electronegativity (attraction to electrons) and there are two oxygen atoms in the carboxylic acid
functional group. These electronegative oxygen atoms help to carry the extra negative charge that is caused when a positive hydrogen atom dissociates. This is why the hydrogen atom in a carboxylic acid is able to dissociate, and the carboxylic acid behaves like an acid.

• Figure 1.25 compares the melting and boiling points of a carboxylic acid with the melting and boiling points of other organic compounds. As you can see, the melting and boiling points of the carboxylic acid are much higher than the melting and boiling points of the other compounds. This is due to the exceptionally strong hydrogen bonding between carboxylic acid molecules.

Carboxylic Acid Derivatives

The strong-smelling compounds you prepared in Investigation 1-A do not fit into any of the organic families you have studied so far. According to their molecular formulas, however, they are isomers of carboxylic acids.
They are esters. Because an ester is obtained by replacing the -OH group of a carboxylic acid with a different group, it is called a derivative of a carboxylic acid. Carboxylic acids have several important derivatives.
In this section, you will study two of these derivatives: esters and amides.

Esters

An ester is an organic compound that has the following functional group:

The general formula for an ester is RCOOR′, where R is a hydrogen atom or a hydrocarbon, and R′ is a hydrocarbon. You can think of an ester as the product of a reaction between a carboxylic acid and an alcohol, as shown in Figure 1.26.

To name an ester, you must recognize that an ester can be thought of as having two distinct parts. The main part of the ester contains the -COO group. This part comes from the parent acid. When numbering the main chain of a carboxylic acid, the carbon atom in the carboxyl group is always given position number 1. The second part of an ester is the alkyl group. To name an ester, follow the steps below.

How to Name an Ester

Step 1 Identify the main part of the ester, which contains the CO group. This part comes from the parent acid. Begin by naming the parent acid.
Step 2 Replace the -oic acid ending of the name of the parent acid with -oate.
Step 3 The second part of an ester is the alkyl group that is attached to the oxygen atom. Name this as you would name any other alkyl group.
Step 4 Put the two names together. Note that esters are named as two words. (See Figure 1.27.)

Table 1.13, on the next page, describes some of the physical properties of esters. As you will see, esters have different physical properties than carboxylic acids, even though esters and carboxylic acids are isomers of each other.

Additional Characteristics of Esters

• Esters often have pleasant odours and tastes, so they are used to produce perfumes and artificial flavours. In fact, the characteristic tastes and smells of many fruits come from esters. (See Figure 1.28.)

Amides

An ester can be thought of as the combination of a carboxylic acid and an alcohol. Similarly, you can think of an amide as the combination of a carboxylic acid and ammonia or an amine. An amide is an organic compound that has a carbon atom double-bonded to an oxygen atom and single-bonded to a nitrogen atom.
Amides have the functional group below:

The general formula for an amide is R-CO-NR2. R can stand for a hydrogen atom or an alkyl group. Figure 1.29 gives some examples of amides.

How to Name an Amide

Step 1 Locate the part of the amide that contains the CO group. Name the parent carboxylic acid that this part derives from. Note: The carbon in the CO group is always given position number 1.
Step 2 Replace the -oic acid ending of the name of the parent acid with the suffix -amide.
Step 3 Decide whether the compound is a primary, secondary, or tertiary amide:
• If there are two hydrogen atoms (and no alkyl groups) attached to the nitrogen atom, the compound is a primary amide and needs no other prefixes.
• If there is one alkyl group attached to the nitrogen atom, the compound is a secondary amide. Name the alkyl group, and give it location letter N- to indicate that it is bonded to the nitrogen atom.
• If there are two alkyl groups, the compound is a tertiary amide. Place the alkyl groups in alphabetical order. Use location letter N- before each group to indicate that it is bonded to the nitrogen atom. If the two groups are identical, use N,N-.
Step 4 Put the name together: prefix + root + suffix.

Additional Characteristics of Amides

When you cut yourself, it is often a good idea to swab the cut with rubbing alcohol to disinfect it. Most rubbing alcohols that are sold in drugstores are based on 2-propanol (common name: isopropanol), C3H8O. You can also swab a cut with a rubbing alcohol based on ethanol, C2H6O. Often it is hard to tell the difference between these two compounds. Both have a sharp smell, and both evaporate quickly. Both are effective at killing bacteria and disinfecting wounds. What is the connection between these compounds? Why is their behaviour so similar?

Functional Groups

Both 2-propanol and ethanol contain the same functional group, an OH (hydroxyl) group, as shown in Figure 1.12. Because ethanol and 2-propanol have the same OH functional group, their behaviour is similar.

The general formula for a family of simple organic compounds is R + functional group. The letter R stands for any alkyl group. (If more than one alkyl group is present, R′ and R′′ are also used.) For example, the general formula ROH refers to any of the following compounds: CH3OH, CH3CH2OH, CH3CH2CH2OH, CH3CH2CH2CH2OH, etc.
Organic compounds are named according to their functional group. Generally, the suffix of a compound’s name indicates the most important functional group in the molecule. For example, the suffix -ene indicates
the presence of a double bond, and the suffix -ol indicates the presence of a hydroxyl group.
Functional groups are a useful way to classify organic compounds, for two reasons: 1. Compounds with the same functional group often have similar physical properties. In the next two sections, you will learn to recognize various functional groups. You will use functional groups to help you predict the physical properties of compounds.
2. Compounds with the same functional group react chemically in very similar ways. In Chapter 2, you will learn how compounds with each functional group react.
Table 1.4, on the next page, lists some of the most common functional groups.

Physical Properties and Forces Between Molecules

Organic compounds that have the same functional group often have similar physical properties, such as boiling points, melting points, and solubilities. Physical properties are largely determined by intermolecular forces, the forces of attraction and repulsion between particles. Three types of intermolecular forces are introduced below. You will examine
these forces further in Chapter 4.
• Hydrogen bonding is a strong intermolecular attraction between the hydrogen atom from an NH, OH, or FH group on one molecule, and a nitrogen, oxygen, or fluorine atom on another molecule.
• The attractive forces between polar molecules are called dipole-dipole interactions. These forces cause polar molecules to cling to each other.
• Dispersion forces are attractive forces that occur between all covalent molecules. These forces are usually very weak for small molecules, but they strengthen as the size of the molecule increases.
The process that is outlined on the next page will help you to predict the physical properties of organic compounds by examining the intermolecular forces between molecules. As you progress through the chapter, referring back to this process will enable you to understand the reasons behind trends in physical properties.

In the following ThoughtLab you will use the process in the box above to predict and compare the physical properties of some organic compounds.

Compounds With Single-Bonded Functional Groups

Alcohols, alkyl halides, ethers, and amines all have functional groups with single bonds. These compounds have many interesting uses in daily life. As you learn how to identify and name these compounds, think about how the intermolecular forces between their molecules affect their properties and uses.

Alcohols

An alcohol is an organic compound that contains the OH functional group. Depending on the position of the hydroxyl group, an alcohol can be primary, secondary, or tertiary. Figure 1.13 gives some examples of alcohols.

Table 1.6 lists some common physical properties of alcohols. As you learned earlier in this chapter, alcohols are polar molecules that experience hydrogen bonding. The physical properties of alcohols depend on these characteristics.

Additional Characteristics of Alcohols
• Alcohols are extremely flammable, and should be treated with caution.
• Most alcohols are poisonous. Methanol can cause blindness or death
when consumed. Ethanol is consumed widely in moderate quantities, but it causes impairment and/or death when consumed in excess.

Alkyl Halides

An alkyl halide (also known as a haloalkane) is an alkane in which one or more hydrogen atoms have been replaced with halogen atoms, such as F, Cl, Br, or I. The functional group of alkyl halides is RX, where X represents a halogen atom. Alkyl halides are similar in structure, polarity, and reactivity to alcohols. To name an alkyl halide, first name the parent hydrocarbon. Then use the prefix fluoro-, chloro-, bromo-, or iodo-, with a position number, to indicate the presence of a fluorine atom, chlorine atom, bromine atom, or iodine atom. The following Sample Problem shows how to name an alkyl halide.

Ethers

Suppose that you removed the H atom from the OH group of an alcohol. This would leave space for another alkyl group to attach to the oxygen atom.

The compound you have just made is called an ether. An ether is an organic compound that has two alkyl groups joined by an oxygen atom.
The general formula of an ether is ROR. You can think of alcohols and ethers as derivatives of the water molecule, as shown in Figure 1.14.
Figure 1.15 gives two examples of ethers.

To name an ether, follow the steps below. The Sample Problem then shows how to use these steps to give an ether its IUPAC name and its common name.

How to Name an Ether
IUPAC Name
Step 1 Choose the longest alkyl group as the parent alkane. Give it an alkane name.
Step 2 Treat the second alkyl group, along with the oxygen atom, as an alkoxy group attached to the parent alkane. Name it by replacing
the -yl ending of the corresponding alkyl group’s name with -oxy. Give it a position number.
Step 3 Put the prefix and suffix together: alkoxy group + parent alkane.

Common Name
Step 1 List the alkyl groups that are attached to the oxygen atom, in alphabetical order.
Step 2 Place the suffix -ether at the end of the name.

Table 1.7 describes some physical properties of ethers. Like alcohols, ethers are polar molecules. Ethers, however, cannot form hydrogen bonds with themselves. The physical properties of ethers depend on these characteristics.

Amines

An organic compound with the functional group -NH2, -NHR, or -NR2 is called an amine. The letter N refers to the nitrogen atom. The letter R refers to an alkyl group attached to the nitrogen. The general formula of an amine is R-NR′2. Amines can be thought of as derivatives of the ammonia molecule, NH3. They are classified as primary, secondary, or tertiary, depending on how many alkyl groups are attached to the nitrogen atom. Note that the meanings of “primary,” “seconday,” and “tertiary” are slightly different from their meanings for alcohols.
Figure 1.16 gives some examples of amines.

To name an amine, follow the steps below. The Sample Problem illustrates how to use these steps to name a secondary amine.

How to Name an Amine
Step 1 Identify the largest hydrocarbon group attached to the nitrogen atom
as the parent alkane.
Step 2 Replace the -e at the end of the name of the parent alkane with
the new ending -amine. Include a position number, if necessary, to
show the location of the functional group on the hydrocarbon chain.
Step 3 Name the other alkyl group(s) attached to the nitrogen atom. Instead
of position numbers, use the letter N- to locate the group(s). (If two
identical alkyl groups are attached to the nitrogen atom, use N,N-.)
This is the prefix.
Step 4 Put the name together: prefix + root + suffix.

Amines are polar compounds. Primary and secondary amines can form hydrogen bonds, but tertiary amines cannot. Table 1.8 lists some common physical properties of amines.

Additional Characteristics of Amines
• Amines are found widely in nature. They are often toxic. Many amines that are produced by plants have medicinal properties.
(See Figure 1.17.)
• Amines with low molecular masses have a distinctive fishy smell. Also, many offensive odours of decay and decomposition are caused by amines. For example, cadavarine, H2NCH2CH2CH2CH2CH2NH2, contributes to the odour of decaying flesh. This compound gets its common name from the word “cadaver,” meaning “dead body.”
• Like ammonia, amines act as weak bases. Since amines are bases, adding an acid to an amine produces a salt. This explains why vinegar and lemon juice (both acids) can be used to neutralize the fishy smell of seafood, which is caused by basic amines.

In this section, you will review the structure and names of hydrocarbons. As you may recall from your previous chemistry studies, hydrocarbons are the simplest type of organic compound. Hydrocarbons are composed
entirely of carbon and hydrogen atoms, and are widely used as fuels. Gasoline, propane, and natural gas are common examples of hydrocarbons. Because they contain only carbon and hydrogen atoms, hydrocarbons are
non-polar compounds.
Scientists classify hydrocarbons as either aliphatic or aromatic. An aliphatic hydrocarbon contains carbon atoms that are bonded in one or more chains and rings. The carbon atoms have single, double, or triple bonds. Aliphatic hydrocarbons include straight chain and cyclic alkanes, alkenes, and alkynes. An aromatic hydrocarbon is a hydrocarbon based on the aromatic benzene group. You will encouter this group later in the section. Benzene is the simplest aromatic compound. Its bonding arrangement results in special molecular stability.

Alkanes, Alkenes, and Alkynes

An alkane is a hydrocarbon that has only single bonds. Alkanes that do not contain rings have the formula CnH2n + 2. An alkane in the shape of a ring is called a cycloalkane. Cycloalkanes have the formula CnH2n. An alkene is a compound that has at least one double bond. Straight-chain alkenes with one double bond have the same formula as cycloalkanes, CnH2n. A double bond involves two pairs of electrons. In a double bond, one pair of electrons forms a single bond and the other pair forms an additional, weaker bond. The electrons in the additional, weaker bond react faster than the electrons in the single bond. Thus, carbon-carbon double bonds are more reactive than carbon-carbon single bonds. When an alkene reacts, the reaction almost always occurs at the site of the double bond.
A functional group is a reactive group of bonded atoms that appears in all the members of a chemical family. Each functional group reacts in a characteristic way. Thus, functional groups help to determine the physical and chemical properties of compounds. For example, the reactive double bond is the functional group for an alkene. In this course, you will encounter many different functional groups. An alkyne is a compound that has at least one triple bond. A straightchain alkyne with one triple bond has the formula CnH2n – 2. Triple bonds are even more reactive than double bonds. The functional group for an alkyne is the triple bond.
Figure 1.7 gives examples of an alkane, a cycloalkane, an alkene, and an alkyne.

General Rules for Naming Organic Compounds

The International Union of Pure and Applied Chemistry (IUPAC) has set standard rules for naming organic compounds. The systematic (or IUPAC) names of alkanes and most other organic compounds follow the same pattern, shown below.

The Root: How Long Is the Main Chain?

The root of a compound’s name indicates the number of carbon atoms in the main (parent) chain or ring. Table 1.2 gives the roots for hydrocarbon chains that are up to ten carbons long. To determine which root to use, count the carbons in the main chain, or main ring, of the compound. If the compound is an alkene or alkyne, the main chain or ring must include the multiple bond.

The Suffix: What Family Does the Compound Belong To?

The suffix indicates the type of compound, according to the functional groups present. (See Table 1.4 on page 22.) As you progress through this chapter, you will learn the suffixes for different chemical families. In your previous chemistry course, you learned the suffixes -ane for alkanes, -ene for alkenes, and -yne for alkynes. Thus, an alkane composed of six carbon atoms in a chain is called hexane. An alkene with three carbons is called propene.

The Prefix: What Is Attached to the Main Chain?

The prefix indicates the name and location of each branch and functional group on the main carbon chain. Most organic compounds have branches, called alkyl groups, attached to the main chain. An alkyl group is obtained
by removing one hydrogen atom from an alkane. To name an alkyl group, change the -ane suffix to -yl. For example, CH3 is the alkyl group that is derived from methane, CH4. It is called the methyl group, taken from the root meth-. Table 1.3 gives the names of the most common alkyl groups.

Read the steps below to review how to name hydrocarbons. Then examine the two Sample Problems that follow.

How to Name Hydrocarbons

Step_1	Find the root: Identify the longest chain or ring in the hydrocarbon. If the hydrocarbon is an alkene or an alkyne, make sure that you include any multiple bonds in the main chain. Remember that the chain does not have to be in a straight line. Count the number of carbon atoms in the main chain to obtain the root. If it is a cyclic compound, add the prefix -cyclo- before the root.
Step_2	Find the suffix: If the hydrocarbon is an alkane, use the suffix -ane. Use -ene if the hydrocarbon is an alkene. Use -yne if the hydrocarbon is an alkyne. If more than one double or triple bond is present, use the prefix di- (2) or tri- (3) before the suffix to indicate the number of multiple bonds.
Step_3	Give a position number to every carbon atom in the main chain. Start from the end that gives you the lowest possible position number for the double or triple bond, if there is one. If there is no double or triple bond, number the compound so that the branches have the lowest possible position numbers.
Step_4	Find the prefix: Name each branch as an alkyl group, and give it a position number. If more than one branch is present, write the names of the branches in alphabetical order. Put the position number of any double or triple bonds after the position numbers and names of the branches, just before the root. This is the prefix. Note: Use the carbon atom with the lowest position number to give the location of a double or triple bond.
Step 5	Put the name together: prefix + root + suffix.

Aromatic Compounds

Chemists do not always use position numbers to describe the branches CHEM that are attached to a benzene ring. When a benzene ring has only two branches, the prefixes ortho-, meta-, and para- are sometimes used instead
of numbers.

Early scientists defined organic compounds as compounds that originate from living things. In 1828, however, the German chemist Friedrich Wohler (1800–1882) made an organic compound called urea, CO(NH2)2, out of an
inorganic compound called ammonium cyanate, NH4CN. Urea is found in the urine of mammals. This was the first time in history that a compound normally made only by living things was made from a non-living substance. Since Wohler had discovered that organic compounds can be made without the involvement of a life process, a new definition was required.
Organic compounds are now defined as compounds that are based on carbon. They usually contain carbon-carbon and carbon-hydrogen bonds.

The Carbon Atom

There are several million organic compounds, but only about a quarter of a million inorganic compounds (compounds that are not based on carbon). Why are there so many organic compounds? The answer lies in the bonding properties of carbon.
As shown in Figure 1.1, each carbon atom usually forms a total of four covalent bonds. Thus, a carbon atom can connect to as many as four other atoms. Carbon can bond to many other types of atoms, including hydrogen, oxygen, and nitrogen.

In addition, carbon atoms can form strong single, double, or triple bonds with other carbon atoms. In a single carbon-carbon bond, one pair of electrons is shared between two carbon atoms. In a double bond, two pairs of electrons are shared between two atoms. In a triple bond, three pairs of electrons are shared between two atoms. Molecules that contain only single carbon-carbon bonds are saturated. In other words, all carbon atoms are bonded to the maximum number of other atoms: four. No more bonding can occur. Molecules that contain double or triple carbon-carbon bonds are unsaturated. The carbon atoms on either side of the double or triple bond are bonded to less than four
atoms each. There is potential for more atoms to bond to each of these carbon atoms.
Carbon’s unique bonding properties allow the formation of a variety of structures, including chains and rings of many shapes and sizes. Figure 1.2 on the next page illustrates some of the many shapes that can be formed from a backbone of carbon atoms. This figure includes examples of three types of structural diagrams that are used to depict organic molecules. (The Concepts and Skills Review contains a further review of these types of structural diagrams.)

Carbon compounds in which carbon forms only single bonds have a different shape than compounds in which carbon forms double or triple bonds. In the following ExpressLab, you will see how each type of bond affects the shape of a molecule.

the shape of a molecule depends on the type of bond. Table 1.1 describes some shapes that you must know for your study of organic chemistry. In Unit 2, you will learn more about why different shapes and angles form around an atom.

Three-Dimensional Structural Diagrams
Two-dimensional structural diagrams of organic compounds, such as condensed structural diagrams and line structural diagrams, work well for flat molecules. As shown in the table above, however, molecules containing single-bonded carbon atoms are not flat.
You can use a three-dimensional structural diagram to draw the tetrahedral shape around a single-bonded carbon atom. In a three-dimensional diagram, wedges are used to give the impression that an atom or group is coming forward, out of the page. Dashed or dotted lines are used to show that an atom or group is receding, or being pushed back into the page. In Figure 1.3, the Cl atom is coming forward, and the Br atom is behind. The two H atoms are flat against the surface of the page.

Molecular Shape and Polarity
The three-dimensional shape of a molecule is particularly important when the molecule contains polar covalent bonds. As you may recall from your previous chemistry course, a polar covalent bond is a covalent bond between two atoms with different electronegativities.
Electronegativity is a measure of how strongly an atom attracts electrons in a chemical bond. The electrons in a polar covalent bond are attracted more strongly to the atom with the higher electronegativity. This atom has a partial negative charge, while the other atom has a partial positive charge. Thus, every polar bond has a bond dipole: a partial negative charge and a partial positive charge, separated by the length of the bond.
Figure 1.4 illustrates the polarity of a double carbon-oxygen bond. Oxygen has a higher electronegativity than carbon. Therefore, the oxygen atom in a carbon-oxygen bond has a partial negative charge, and the carbon atom
has a partial positive charge.

Other examples of polar covalent bonds include CO, OH, and NH. Carbon and hydrogen attract electrons to almost the same degree. Therefore, when carbon is bonded to another carbon atom or to a hydrogen atom, the bond is not usually considered to be polar. For example, CC bonds are considered to be non-polar.

Predicting Molecular Polarity
A molecule is considered to be polar, or to have a molecular polarity, when the molecule has an overall imbalance of charge. That is, the molecule has a region with a partial positive charge, and a region with a partial negative charge. Surprisingly, not all molecules with polar bonds are polar molecules. For example, a carbon dioxide molecule has two
polar CO bonds, but it is not a polar molecule. On the other hand, a water molecule has two polar OH bonds, and it is a polar molecule.
How do you predict whether or not a molecule that contains polar bonds has an overall molecular polarity? To determine molecular polarity, you must consider the shape of the molecule and the bond dipoles within the
molecule.
If equal bond dipoles act in opposite directions in three-dimensional space, they counteract each other. A molecule with identical polar bonds that point in opposite directions is not polar. Figure 1.5 shows two examples, carbon dioxide and carbon tetrachloride. Carbon dioxide, CO2, has two polar CO bonds acting in opposite directions, so the molecule
is non-polar. Carbon tetrachloride, CCl4, has four polar CCl bonds in a tetrahedral shape. You can prove mathematically that four identical dipoles, pointing toward the vertices of a tetrahedron, counteract each other exactly. (Note that this mathematical proof only applies if all four bonds are identical.) Therefore, carbon tetrachloride is also non-polar.

If the bond dipoles in a molecule do not counteract each other exactly, the molecule is polar. Two examples are water, H2O, and chloroform, CHCl3, shown in Figure 1.6. Although each molecule has polar bonds, the bond dipoles do not act in exactly opposite directions. The bond dipoles do not counteract each other, so these two molecules are polar.

The steps below summarize how to predict whether or not a molecule is polar. The Sample Problem that follows gives three examples.
Note: For the purpose of predicting molecular polarity, you can assume that CH bonds are non-polar. In fact, they have a very low polarity.
Step 1 Does the molecule have polar bonds? If your answer is no, see below. If your answer is yes, go to step 2.
If a molecule has no polar bonds, it is non–polar. Examples: CH3CH2CH3, CH2CH2
Step 2 Is there more than one polar bond? If your answer is no, see below. If your answer is yes, go to step 3.
If a molecule contains only one polar bond, it is polar. Examples: CH3Cl, CH3CH2CH2Cl
Step 3 Do the bond dipoles act in opposite directions and counteract each other? Use your knowledge of three-dimensional molecular shapes to help you answer this question. If in doubt, use a molecular model to help you visualize the shape of the molecule.
If a molecule contains bond dipoles that do not counteract each other, the molecule is polar. Examples: H2O, CHCl3
If the molecule contains dipoles that counteract each other, the molecule is non–polar. Examples: CO2, CCl4