Saturated Hydrocarbons

What is a functional group?

A group of atoms in an organic molecule responsible for its characteristic chemical properties and reactions

What are alkanes?

Hydrocarbons containing only carbon and hydrogen atoms

Why are alkanes called saturated hydrocarbons?

• contain the maximum possible number of hydrogens per carbon

• only contain C–C and C–H single bonds

What does “aliphatic” mean?

Refers to hydrocarbons derived from the Greek aleiphas meaning “fat”

What bonds do alkanes contain?

Only strong σ bonds formed by sp³–sp³ orbital overlap

What is the general formula for alkanes?

C_nH_2n+2

What is an alkyl group?

A partial structure formed when one hydrogen is removed from an alkane

How are carbons classified in alkanes?

• Primary (1°)

• Secondary (2°)

• Tertiary (3°)

• Quaternary (4°)

What is the relative reactivity order of C–H bonds?

Quaternary < Tertiary < Secondary < Primary (reactivity increases)

What are conformers?

Different spatial arrangements due to rotation around a C–C single bond

What is the most stable ethane conformation?

• Staggered (lowest energy)

• All six C-H bonds are as far away from one another as possible

Least stable ethane conformation?

• Eclipsed (highest energy)

• All six C-H bonds are as close to one another as possible

Why is cyclohexane not flat?

Due to free rotation about the various C-C σ bonds

What are axial hydrogens?

Hydrogens perpendicular to the ring (parallel to ring axis)

What are equatorial hydrogens?

Hydrogens lying roughly in the plane of the ring

How many axial and equatorial hydrogens in chair cyclohexane?

6 axial and 6 equatorial

Are alkanes polar or non-polar?

Non-polar

Alkanes in water

Hydrophobic

Why are alkanes insoluble in water?

They cannot form hydrogen bonds

Alkane solubility in lipids

Lipophilic

Why are alkanes lipid soluble?

Due to strong van der Waals forces of attraction between hydrocarbons

How do alkanes behave in biological systems?

They partition rapidly into lipid-rich areas (e.g. brain)

How are gaseous alkanes absorbed?

Via the lungs (e.g. cyclopropane)

How does chain length affect drugs?

Alters lipophilicity, absorption, distribution, and excretion

Effect of branching on lipophilicity?

Branching decreases lipophilicity

Why does branching reduce lipophilicity?

The molecule becomes more compact and disrupts water H-bonding less

Can branching affect pharmacological activity?

Yes- especially if the chain interacts with receptors

Example drugs where branching changes activity?

Promethazine (antihistamine) vs Promazine (antipsychotic)

Why are alkanes chemically unreactive?

They contain only strong C–C and C–H sigma bonds

Do alkanes react with acids or bases?

No

What happens when alkanes combust?

They react with oxygen to form CO₂ and H₂O

Is combustion exothermic or endothermic?

Exothermic

Why are alkanes good fuels?

They release large amounts of energy on combustion

What is the other major reaction of alkanes besides combustion?

Halogenation

What conditions are required for halogenation?

UV light or high temperature

What mechanism does halogenation follow?

Free radical mechanism

What is a radical?

A species with an unpaired electron

How are chlorine radicals formed?

UV light causes photolysis of Cl₂ → 2 Cl· radicals

What is a radical?

A species with an unpaired electron

Steps of radical halogenation of alkanes

• Initiation

• Propagation

• Termination

What happens if excess Cl₂ is present?

Multiple substitutions occur

Possible products of methane chlorination?

• CH₃Cl

• CH₂Cl₂

• CHCl₃ (chloroform)

• CCl₄ (carbon tetrachloride)