Alkyl Foundations

Alkyl Group

A substituent formed conceptually by removing one hydrogen atom from an alkane.

Formation of an Alkyl Group

Remove one hydrogen atom from an alkane, leaving a carbon with an available point of attachment to another atom or group.

General Formula of an Alkyl Group

CₙH₂ₙ₊₁, where n is the number of carbon atoms in the alkyl group.

Why does an alkyl group have the general formula CₙH₂ₙ₊₁?

An alkane has the formula CₙH₂ₙ₊₂; removing one hydrogen to form an alkyl group gives CₙH₂ₙ₊₁.

Alkane-to-Alkyl Naming Rule

Replace the -ane ending of the corresponding alkane name with -yl.

-yl

The suffix used in the name of an alkyl group.

Point of Attachment

The position on an alkyl group where the removed hydrogen was located and where the group can form a bond to the rest of a molecule.

Free Valence of an Alkyl Group

The available bond at the point of attachment that allows an alkyl group to bond to another atom or group.

R

A general symbol used in organic chemistry to represent an unspecified alkyl group or other carbon-containing group.

Methyl Group

The one-carbon alkyl group CH₃–, formed by removing one hydrogen from methane.

Formation of Methyl

Methane (CH₄) − H → methyl (CH₃–).

Methyl Formula

CH₃–

Methyl Abbreviation

Me

Ethyl Group

The two-carbon alkyl group CH₃CH₂–, formed by removing one hydrogen from ethane.

Formation of Ethyl

Ethane (CH₃CH₃) − H → ethyl (CH₃CH₂–).

Ethyl Formula

CH₃CH₂–

Ethyl Abbreviation

Et

Propyl Group

The straight-chain three-carbon alkyl group CH₃CH₂CH₂–, formed by removing a terminal hydrogen from propane.

Formation of Propyl

Propane (CH₃CH₂CH₃) − H from an end carbon → propyl (CH₃CH₂CH₂–).

Propyl Formula

CH₃CH₂CH₂–

Propyl Abbreviation

Pr

Butyl Group

The straight-chain four-carbon alkyl group CH₃CH₂CH₂CH₂–, formed by removing a terminal hydrogen from butane.

Formation of Butyl

Butane (CH₃CH₂CH₂CH₃) − H from an end carbon → butyl (CH₃CH₂CH₂CH₂–).

Butyl Formula

CH₃CH₂CH₂CH₂–

Butyl Abbreviation

Bu

Pentyl Group

The straight-chain five-carbon alkyl group CH₃(CH₂)₄–, formed by removing a terminal hydrogen from pentane.

Formation of Pentyl

Pentane − H from an end carbon → pentyl.

Pentyl Formula

CH₃(CH₂)₄–

Alkane vs. Alkyl Group

An alkane is a complete saturated hydrocarbon, whereas an alkyl group is derived from an alkane by removing one hydrogen and acts as part of a larger molecule.

Methane → Methyl

CH₄ → CH₃–

Ethane → Ethyl

C₂H₆ → C₂H₅–

Propane → Propyl

C₃H₈ → C₃H₇–

Butane → Butyl

C₄H₁₀ → C₄H₉–

Pentane → Pentyl

C₅H₁₂ → C₅H₁₁–

1-Carbon Alkyl Group

Methyl

2-Carbon Alkyl Group

Ethyl

3-Carbon Straight-Chain Alkyl Group

Propyl

4-Carbon Straight-Chain Alkyl Group

Butyl

5-Carbon Straight-Chain Alkyl Group

Pentyl

Relationship Between Parent Alkane and Alkyl Names

The carbon-count root remains the same while the ending changes from -ane to -yl, such as ethane → ethyl and butane → butyl.

Does removing hydrogen from different carbons always produce the same alkyl group?

No. For some alkanes, removing hydrogen from different nonequivalent carbon positions produces different alkyl groups.

Why can propane produce more than one alkyl group?

Propane has nonequivalent terminal and middle carbon positions, so removing hydrogen from different positions can produce different points of attachment.

Two Alkyl Groups Derived from Propane

Propyl and isopropyl.

Why can butane produce more than one alkyl group?

Removing hydrogen from different nonequivalent carbon positions can create alkyl groups with different points of attachment.

Role of Alkyl Groups in IUPAC Nomenclature

Alkyl groups attached to a parent hydrocarbon are named as substituents and appear as prefixes in the complete IUPAC name.

Alkyl Substituent

An alkyl group attached to the parent structure of an organic compound.

Parent vs. Alkyl Substituent

The parent provides the main hydrocarbon name, while an attached alkyl group is named as a substituent prefix.