Organic Chemistry

104 CHAPTER 2 An Introduction to Organic Compounds

position. The cis isomer will have one substituent in the equatorial position and one

substituent in the axial position. Trans-fusedcyclohexane rings, therefore, are more

stable than cis-fusedcyclohexane rings.

H

H

H

H

trans-decalin

trans-fused rings

more stable

cis-decalin

cis-fused rings

less stable

equatorial

equatorial

equatorial

axial

Summary

Alkanesare hydrocarbonsthat contain only single bonds.

Their general molecular formula is Constituti-

onal isomershave the same molecular formula, but their

atoms are linked differently. Alkanes are named by deter-

mining the number of carbons in their parent hydrocar-

bon—the longest continuous chain. Substituentsare listed

in alphabetical order, with a number to designate their posi-

tion on the chain. When there is only a substituent, the sub-

stituent gets the lowest possible number; when there is only

a functional group suffix, the functional group suffix gets

the lowest possible number; when there is both a functional

group suffix and a substituent, the functional group suffix

gets the lowest possible number. The functional groupis

the center of reactivity in a molecule.

Alkyl halidesand ethersare named as substituted alkanes.

Alcoholsand aminesare named using a functional group suf-

fix. Systematic names can contain numbers; common

namesnever do. A compound can have more than one name,

but a name must specify only one compound. Whether alkyl

halides or alcohols are primary,secondary, or tertiaryde-

pends on whether the X (halogen) or OH group is bonded to a

primary, secondary, or tertiary carbon. A primary carbonis

bonded to one carbon, a secondary carbonis bonded to two

carbons, and a tertiary carbonis bonded to three carbons.

Whether amines are primary,secondary, or tertiaryde-

pends on the number of alkyl groups bonded to the nitrogen.

Compounds with four alkyl groups bonded to nitrogen are

called quaternary ammonium salts.

The oxygen of an alcohol has the same geometry it has

in water; the nitrogen of an amine has the same geometry it

has in ammonia. The greater the attractive forces between

molecules—van der Waals forces,dipole–dipole interac-

tions, hydrogen bonds—the higher is the boiling pointof

the compound. A hydrogen bondis an interaction between

a hydrogen bonded to an O, N, or F and the lone pair of an

O, N, or F in another molecule. The boiling point increases

with increasing molecular weight of the homolog. Branch-

CnH 2 n+ 2.

ing lowers the boiling point. Polarizabilityindicates the

ease with which an electron cloud can be distorted: Larger

atoms are more polarizable.

Polar compounds dissolve in polar solvents, and

nonpolar compoundsdissolve in nonpolar solvents. The

interaction between a solvent and a molecule or an ion dis-

solved in that solvent is called solvation. The oxygen of an

alcohol or an ether can drag about three or four carbons into

solution in water.

Rotation about a bond results in two extreme

conformations that rapidly interconvert:staggered and

eclipsed. A staggered conformer is more stable than

an eclipsed conformer because of torsional strain—

repulsion between pairs of bonding electrons. There can be

two different staggered conformers: The anti conformeris

more stable than the gauche conformerbecause of steric

strain—repulsion between the electron clouds of atoms or

groups. The steric strain in a gauche conformer is called a

gauche interaction.

Five- and six-membered rings are more stable than

three- and four-membered rings because of angle strain

that results when bond angles deviate from the ideal bond

angle of 109.5°. In a process called ring flip, cyclohexane

rapidly interconverts between two stable chair conforma-

tions. Bonds that are axialin one chair conformer are

equatorialin the other and vice versa. The chair conformer

with a substituent in the equatorial position is more stable,

because there is more room in an equatorial position. A

substituent in an axial position experiences unfavorable

1,3-diaxial interactions. In the case of disubstituted cyclo-

hexanes, the more stable conformer will have its larger sub-

stituent in the equatorial position. A cis isomerhas its two

substituents on the same side of the ring; a trans isomer

has its substituents on opposite sides of the ring. Cis and

trans isomers are called geometric isomersor cis–trans

isomers. Cyclohexane rings are more stable if they are

trans fusedthan cis fused.

C¬C

BRUI02-060_108r4 20-03-2003 11:48 AM Page 104