Branched Alkyl Groups

Branched Alkyl Group

An alkyl group whose carbon skeleton is branched rather than forming one continuous unbranched chain.

Alkyl-Group Isomerism

The existence of different alkyl groups having the same molecular formula but different carbon connectivity or different points of attachment.

Point of Attachment in an Alkyl Group

The carbon atom through which an alkyl group bonds to the rest of a molecule.

Why can one alkane produce different alkyl groups?

Removing hydrogen from nonequivalent carbon atoms can create different points of attachment and therefore different alkyl groups.

Propyl Group

The straight-chain three-carbon alkyl group CH₃CH₂CH₂–, obtained by removing a hydrogen from an end carbon of propane.

Isopropyl Group

The branched three-carbon alkyl group (CH₃)₂CH–, obtained by removing a hydrogen from the middle carbon of propane.

Propyl Formula

CH₃CH₂CH₂–

Isopropyl Formula

(CH₃)₂CH–

Propyl vs. Isopropyl

Propyl attaches through an end carbon of the three-carbon skeleton, whereas isopropyl attaches through the middle carbon.

How many distinct alkyl groups can be derived from propane?

Two: propyl and isopropyl.

Formation of Propyl from Propane

Remove a hydrogen from either terminal carbon of propane to form CH₃CH₂CH₂–.

Formation of Isopropyl from Propane

Remove the hydrogen from the middle carbon of propane to form (CH₃)₂CH–.

Why do the two end carbons of propane produce the same propyl group?

The two terminal carbons are equivalent by symmetry, so removing hydrogen from either end produces the same alkyl group.

Butyl Groups

The four distinct four-carbon alkyl groups: butyl, sec-butyl, isobutyl, and tert-butyl.

How many distinct four-carbon alkyl groups exist?

Four: butyl, sec-butyl, isobutyl, and tert-butyl.

Butyl (n-Butyl)

The straight-chain four-carbon alkyl group CH₃CH₂CH₂CH₂–, attached through a terminal carbon.

n-Butyl Formula

CH₃CH₂CH₂CH₂–

sec-Butyl

A four-carbon alkyl group derived from straight-chain butane by removing a hydrogen from a secondary carbon; CH₃CH(–)CH₂CH₃.

sec-Butyl Formula

CH₃CH(–)CH₂CH₃

n-Butyl vs. sec-Butyl

Both have an unbranched four-carbon skeleton, but n-butyl attaches through a terminal carbon whereas sec-butyl attaches through an internal secondary carbon.

Formation of n-Butyl from Butane

Remove a hydrogen from a terminal carbon of n-butane.

Formation of sec-Butyl from Butane

Remove a hydrogen from an internal carbon of n-butane.

Isobutyl

The branched four-carbon alkyl group (CH₃)₂CHCH₂–, attached through a CH₂ carbon.

Isobutyl Formula

(CH₃)₂CHCH₂–

tert-Butyl

The branched four-carbon alkyl group (CH₃)₃C–, attached through the central carbon.

tert-Butyl Formula

(CH₃)₃C–

Isobutyl vs. tert-Butyl

Both have a branched four-carbon skeleton, but isobutyl attaches through a terminal CH₂ carbon whereas tert-butyl attaches through the central carbon.

Formation of Isobutyl from Isobutane

Remove a hydrogen from one of the equivalent CH₃ groups of isobutane (2-methylpropane), producing (CH₃)₂CHCH₂–.

Formation of tert-Butyl from Isobutane

Remove the hydrogen from the central carbon of isobutane (2-methylpropane), producing (CH₃)₃C–.

Butane-Derived Alkyl Groups

n-Butyl and sec-butyl.

Isobutane-Derived Alkyl Groups

Isobutyl and tert-butyl.

Straight-Skeleton C₄ Alkyl Groups

n-Butyl and sec-butyl.

Branched-Skeleton C₄ Alkyl Groups

Isobutyl and tert-butyl.

Primary Attachment Carbon

A point-of-attachment carbon bonded directly to only one other carbon atom.

Secondary Attachment Carbon

A point-of-attachment carbon bonded directly to two other carbon atoms.

Tertiary Attachment Carbon

A point-of-attachment carbon bonded directly to three other carbon atoms.

Attachment Type of n-Butyl

Primary.

Attachment Type of sec-Butyl

Secondary.

Attachment Type of Isobutyl

Primary.

Attachment Type of tert-Butyl

Tertiary.

Meaning of sec- in sec-Butyl

The group is attached to the rest of the molecule through a secondary carbon.

Meaning of tert- in tert-Butyl

The group is attached to the rest of the molecule through a tertiary carbon.

Does iso- mean that the attachment carbon is secondary?

No. iso- describes a characteristic branching pattern and does not directly specify whether the attachment carbon is primary, secondary, or tertiary.

Is Isobutyl a Primary Alkyl Group?

Yes. Although its carbon skeleton is branched, its point-of-attachment carbon is bonded to only one other carbon.

Is Isopropyl a Secondary Alkyl Group?

Yes. Its point-of-attachment carbon is bonded directly to two other carbon atoms.

Is tert-Butyl a Tertiary Alkyl Group?

Yes. Its point-of-attachment carbon is bonded directly to three other carbon atoms.

Common Name

A traditional name widely used for a compound or substituent rather than a name derived entirely from systematic IUPAC rules.

Why are common alkyl-group names important?

Names such as isopropyl, sec-butyl, isobutyl, and tert-butyl are widely used and are important to recognize in organic chemistry.

Recognizing Propyl

Look for a three-carbon unbranched group attached through an end carbon: CH₃CH₂CH₂–.

Recognizing Isopropyl

Look for a point-of-attachment carbon bonded to two CH₃ groups: (CH₃)₂CH–.

Recognizing n-Butyl

Look for a four-carbon unbranched chain attached through an end carbon: CH₃CH₂CH₂CH₂–.

Recognizing sec-Butyl

Look for a four-carbon unbranched skeleton attached through an internal carbon: CH₃CH(–)CH₂CH₃.

Recognizing Isobutyl

Look for the branched pattern (CH₃)₂CHCH₂–, with attachment through the CH₂ group.

Recognizing tert-Butyl

Look for a point-of-attachment carbon bonded to three CH₃ groups: (CH₃)₃C–.