Organic Chemistry

Section 2.1 Nomenclature of Alkyl Substituents 65

There are four alkyl groups that contain four carbon atoms. The butyl and isobutyl

groups have a hydrogen removed from a primary carbon. A sec-butyl group has a hy-

drogen removed from a secondary carbon (sec-, often abbreviated s-, stands for sec-

ondary), and a tert-butyl group has a hydrogen removed from a tertiary carbon (tert-,

sometimes abbreviated t-, stands for tertiary). A tertiary carbonis a carbon that is

bonded to three other carbons. Notice that the isobutyl group is the only group with an

iso structural unit.

A name of a straight-chain alkyl group often has the prefix “n”(for “normal”), to

emphasize that its carbon atoms are in an unbranched chain. If the name does not have

a prefix such as “n”or “iso,”it is assumed that the carbons are in an unbranched chain.

butyl bromide pentyl fluoride

or or

n-butyl bromide n-pentyl fluoride

Like the carbons, the hydrogens in a molecule are also referred to as primary, sec-

ondary, and tertiary. Primary hydrogensare attached to primary carbons,secondary

hydrogensto secondary carbons, and tertiary hydrogensto tertiary carbons.

Because a chemical name must specify only one compound, the only time you

will see the prefix “sec”is in sec-butyl. The name “sec-pentyl”cannot be used be-

cause pentane has two different secondary carbon atoms. Therefore, there are two

different alkyl groups that result from removing a hydrogen from a secondary car-

bon of pentane. Because the name would specify two different compounds, it is not

a correct name.

CH 3 CHCH 2 CH 2 CH 3

Cl

Both alkyl halides have five carbon atoms with a chloride attached to a

secondary carbon, so both compounds would be named sec-pentyl chloride.

CH 3 CH 2 CHCH 2 CH 3

Cl

CH 3 CH 2 CH 2 CH 2 Br CH 3 CH 2 CH 2 CH 2 F

CH 3 CH 2 CH 2 CH 2

a butyl group

CH 3 CHCH 2

CH 3

an isobutyl group

CH 3 CH 2 CH

CH 3

a sec-butyl group

CH 3 C

CH 3

CH 3

a tert-butyl group

a primary carbon a primary carbon a secondary carbon a tertiary carbon

butyl alcohol

or

n-butyl alcohol

isobutyl alcohol sec-butyl alcohol

or

s-butyl alcohol

tert-butyl alcohol

or

t-butyl alcohol

CH 3 COH

CH 3

CH 3

CH 3 CH 2 CH 2 CH 2 OH

primary hydrogens

CH 3 CHCH 2 OH

CH 3

tertiary hydrogen

CH 3 CH 2 CHOH

CH 3

secondary hydrogens

A primary carbon is bonded to one

carbon, a secondary carbon is bonded to

two carbons, and a tertiary carbon is

bonded to three carbons.

3-D Molecules:

n-Butyl alcohol; sec-Butyl

alcohol; tert-Butyl alcohol

Tutorial:

Alkyl group nomenclature

Tutorial:

Degree of alkyl substitution

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