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Alkanes, Alkyl Groups, and Functional Groups: Nomenclature and Properties

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Alkanes and Alkyl Groups

General Formula and Naming of Alkanes

Alkanes are saturated hydrocarbons with the general formula . They consist only of single bonds between carbon atoms and are the simplest type of organic molecules.

  • Alkane Structure: Linear or branched chains of carbon atoms, fully saturated with hydrogen.

  • Alkyl Groups: Derived from alkanes by removing one hydrogen atom, denoted as R-.

  • Naming Convention: Use Greek prefixes to indicate the number of carbons (meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-).

Table: Alkane and Alkyl Group Structures

Alkane Structure

Name

Alkyl Piece Structure

Name

No. of C's

CH4

methane

H3C–

methyl

1

CH3CH3

ethane

H3CCH2–

ethyl

2

CH3CH2CH3

propane

H3CCH2CH2–

propyl

3

CH3CH2CH2CH3

butane

H3CCH2CH2CH2–

butyl

4

CH3CH2CH2CH2CH3

pentane

H3CCH2CH2CH2CH2–

pentyl

5

CH3CH2CH2CH2CH2CH3

hexane

H3CCH2CH2CH2CH2CH2–

hexyl

6

CH3CH2CH2CH2CH2CH2CH3

heptane

H3CCH2CH2CH2CH2CH2CH2–

heptyl

7

CH3CH2CH2CH2CH2CH2CH2CH3

octane

H3CCH2CH2CH2CH2CH2CH2CH2–

octyl

8

CH3CH2CH2CH2CH2CH2CH2CH2CH3

nonane

H3CCH2CH2CH2CH2CH2CH2CH2CH2–

nonyl

9

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

decane

H3CCH2CH2CH2CH2CH2CH2CH2CH2CH2–

decyl

10

Additional info: Common alkyl group names for longer chains include lauryl (12C), myristyl (14C), cetyl (16C), stearyl (18C), arachidyl (20C).

Alkane Properties

Alkanes are nonpolar molecules, making them hydrophobic (water-fearing) and insoluble in water. Their physical properties depend on chain length and branching.

  • Boiling Point: Longer chains have higher boiling points due to increased van der Waals forces.

  • Solubility: Alkanes are insoluble in water but soluble in nonpolar solvents.

  • Fatty Acids: Long alkyl chains are found in fatty acids, which are important in biological systems.

  • Energy Content: Alkanes are energy-rich; for example, complete oxidation of triacylglycerol yields about 38 kJ/g.

Constitutional Isomers

Constitutional isomers are compounds with the same molecular formula but different connectivity of atoms.

  • Definition: Isomers with the same number and type of atoms, but different arrangement.

  • Example: Four constitutional isomers exist for (butane).

  • Key Point: Not all alkyl groups are linear; branching can occur.

Alkane Formula Representations

Alkanes can be represented in several ways:

  • Formula:

  • Condensed: CH3(CH2)nCH3

  • Highly Condensed: CH3CH2CH3

  • Skeletal: Lines representing carbon-carbon bonds, omitting hydrogens.

Common Names for Alkyl Groups

Group

Name

Skeletal Representation

CH3CH(CH3)–

isopropyl

Y-shaped structure

CH3CH(CH3)CH2–

isobutyl

Branched structure

CH3C(CH3)2–

tert-butyl

Central carbon with three methyl groups

(CH3)2CH–

sec-butyl

Secondary carbon attached to two methyls

Nomenclature of Alkanes

How to Name Alkanes

  1. Find the longest chain: The parent chain determines the base name.

  2. Name and number substituents: Number from the end that gives the lowest set of locants.

  3. Alphabetize substituents: Ignore hyphenated prefixes (sec-, tert-, di-, tri-, tetra-, penta-).

  4. Functional Groups: Name molecules by their key functional groups (e.g., alcohol, ketone, ester).

  5. Lowest Number Rule: Assign the lowest possible numbers to substituents.

  6. Simplest Substituent Names: Use the simplest name for substituents over complex ones.

Nomenclature of Cycloalkanes

  • Count the number of carbons in the ring.

  • Name using the prefix for the number of carbons and the suffix -cyclo- (e.g., cyclopentane).

  • Number substituents to give the lowest possible numbers.

  • If multiple substituents, number to give the lowest set of locants; if a tie, alphabetize.

Other Functional Groups

Recognizing functional groups is essential for organic chemistry nomenclature and reactivity.

  • Alkyl Halides: R–X, where X = F, Cl, Br, I

  • Ethers: R–O–R'

  • Alcohols: R–OH

  • Ketones: R–CO–R'

  • Carboxylic Acids: R–COOH

  • Carboxylic Esters: R–COO–R'

  • Carboxylic Amides: R–CONH2

Unsaturated Hydrocarbon Functional Groups

  • Alkene: Contains C=C double bonds

  • Alkyne: Contains C≡C triple bonds

  • Benzene/Aromatic: Contains a conjugated ring system (aromatic ring)

Practice Examples

Complex molecules may contain multiple functional groups, such as oxycodone, cocaine, and fentanyl.

Special Topic: Degree of Unsaturation

Definition and Calculation

Degree of unsaturation indicates the number of rings and/or pi bonds in a molecule.

  • One pi bond or one ring = one degree of unsaturation

  • Two pi bonds, two rings, or one ring + one pi bond = two degrees of unsaturation

  • Formula:

  • Example: (benzene) has four degrees of unsaturation (three double bonds and one ring)

Special Designations for Types of Carbons

  • Primary (1°) Carbon: Attached to one other carbon

  • Secondary (2°) Carbon: Attached to two other carbons

  • Tertiary (3°) Carbon: Attached to three other carbons

  • Quaternary (4°) Carbon: Attached to four other carbons

Summary Table: Types of Carbons

Type

Definition

Primary (1°)

Carbon attached to one other carbon

Secondary (2°)

Carbon attached to two other carbons

Tertiary (3°)

Carbon attached to three other carbons

Quaternary (4°)

Carbon attached to four other carbons

Additional info: The skeletal structure is often used to simplify complex organic molecules, omitting hydrogens for clarity.

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