Common Functional Groups

In addition to being replaced by simple single-atom substituents like CI and F, the hydrogen atoms in alkanes and alkenes can be replaced by more complex "attachments" called functional groups—these are typically headed by oxygen or nitrogen atoms. The common functional groups are listed in Table 2. The simplest such polyatomic group is —O — H, usually simply shown as — OH; it is called the hydroxyl group. Compounds that correspond to alkanes or alkenes with the hydrogen of one C — H bond replaced by an — OH group are called alcohols. Familiar examples are methyl alcohol or methanol (also called wood alcohol) and ethyl alcohol or ethanol (grain alcohol):

H H H

methanol, CH3OH ethanol, CH3CH2OH

The use of alcohols as fuels is discussed in Chapter 8.

l,\;?l :: 1 Some Common Functional Groups

Name of Compound Type

Functional Group

Chloride

—CI

Fluoride

—F

Alcohol

—OH

Ether

—O—

Aldehyde

O

Carboxylic acid

^OH

Amine

-<

Compounds called ethers contain an oxygen atom connected on both sides to a carbon atom or chain:

In more formal names for such compounds, the —OCH3 group is known as methoxy, and the —OCH?—CH3 group is known as ethoxy, so that dimethyl ether would be named methoxymethane. The use of ethers as gasoline fuel additives is discussed in Chapter 8,

There are organic compounds analogous to alcohols and ethers in which sulfur occurs in the position otherwise occupied by oxygen. The prefix thio- is used to denote this substitution; thus we have thioalcohols, or just thiols, such as CH3SH, and thioethers, such as CH3 — S—CH<.

As discussed in Chapter 3, carbon-oxygen double bonds are found in some organic molecules. Molecules that contain the H—C=0 group bonded to hydrogen or to a carbon are known as aldehydes; the important examples encountered in polluted air are formaldehyde, H2C=0, and acetaldehyde, CHjCCH) ==0. (Atoms or groups shown inside parentheses are bonded to the preceding carbon but do not themselves participate in the bond displayed next in the formula.)

H H,C

H H

formaldehyde, H2CO acetaldehyde, CH3CHO

If the C=0 group is connected to an —OH group, the system is called a carboxylic acid; examples are formic acid and acetic acid:

formic acid, HCOOH acetic acid, CH3CDOH

If the hydrogen atom of the —OH group is replaced by an organic group, the compound is called an ester.

Groups headed by nitrogen atoms are known as amino groups; they are found attached to carbon chains in some organic molecules. Compounds in which the amino group is bonded to a hydrocarbon chain are called amines. Note that nitrogen atoms form a total of three bonds, some (or all) of which can be directed to carbons. Two examples are:

H H H

methylamme, CH3NH2 dimethylamtne, (CH3)2NH

Molecules that contain both the carboxylic acid group and an amino group are called amino acids. They are the most important groups in proteins, which include the enzymes that accelerate specific biological reactions. The amino acid called cysteine is illustrated below; notice that it contains a thiol group as well as an amino and a carboxylic acid group.

cysteine

Alcohols, acids, and amines that contain short carbon chains are quite soluble in water. The reason is that molecules of these three types contain O—H or N — H bonds, which possess a hydrogen atom that is partially depleted of electron density by the highly electronegative atom (O or N) to which it is bonded. The partial positive charge 8+ of the hydrogen is attracted to regions of unbonded electron density—lone pairs—on atoms of adjacent molecules:

hydrogen bond

Such interactions are called hydrogen bonds; the forces holding the two atoms together—and therefore also holding together the two molecules to which the atoms belong—are not nearly as strong as those of a regular bond within a molecule, but they are much stronger than the forces that operate between molecules in hydrocarbons. Water molecules illustrate this situation. They stick together because each hydrogen atom is hydrogen-bonded to the lone pair of the H20 molecule closest to it. The attraction between H20 molecules due to these interactions results in a relatively high boiling point for liquid water, much higher than anticipated for a molecule of its mass. For a (nonionic) substance to be freely soluble in water, these secondary bonds between adjacent water molecules must be replaced by similar interactions between the substance and the water molecules. Consequently, molecules that contain N-H or O-H bonds and a short chain of carbons are soluble in water because the hydrogen bonds they form with H20 molecules replace those that are broken when the substance is incorporated into the liquid.

Hydrogen atoms bonded to carbon cannot form hydrogen bonds with water molecules, since the carbon is not sufficiently electronegative to produce much of a positive charge on a hydrogen atom bonded to it. In addition, there are no lone pairs of electrons on the carbon atoms. Consequently, there is no driving force that can disrupt the extensive network of hydrogen bonding within liquid water in order to incorporate a large number of molecules of hydrocarbons or chlorinated organic molecules. In both hydrocarbon and chlorinated organic molecules, all the hydrogens are bonded to carbon; for this reason, such molecules are not very soluble in water. Even molecules with one O — H or N — 11 group and many carbon atoms are insoluble in water, since their overall character is dominated by the large number of carbons. The forces of attraction that do exist between organic molecules that contain no hydrogen-bonding capacity are quite nonspecific and nondirectional; consequently, different hydrocarbons are quite soluble in each other, and organochlorine molecules are soluble in hydrocarbons. We can restate the familiar generalization that "like dissolves like": Compounds tend to dissolve in other substances having the same types of intermolecular interactions.

PROBLEM 6

Write the structural formulas and symbolic diagrams for each of the following: (a) ethyl alcohol, (b) ethylamine, (c) acetic acid (the carboxylic acid with a methyl group bonded to the carbon).

Continue reading here: Rings of Carbon Atoms

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