Organic chem: Alkenes

Rules for naming alkenes

• The position of the c=c must be specified in the name

• The alkene is always given priority when applying numbers over alkly groups

• The longest carbon chain becomes the one with the alkene

• Multiple alkenes in the same molecule must be names using di/tri/tetra/penta

• Always include whether it is a E/Z isomer in the name

What does a double bond consist of?

A sigma bond and a π bond

How are π bonds formed?

• They form due to the sideways overlap of adjacent p-orbitals

• This creates a region of increased electron density above and below the σ-bond

Shape and bond angle of alkenes

• Trigonal planar shape

• Bond angle of 120°

Why are alkenes more reactive than alkanes?

The π bond is weaker than the σ-bond

Why do alkenes perform addition reactions?

Due to the weaker π bond

What are the 4 types of addition reactions?

• Halogenation

• Hydrogenation

• Hydrogen halide

• Hydration

Halogenation- conditions and eg

Conditions: any halogen, room temp, room pressure

Hydrogenation- conditions and eg

Conditions: hydrogen, 150°C, nickel catalyst

Hydrogen halide- conditions and eg

Conditions: any hydrogen halide, room temp, room pressure

Hydration- conditions and eg

Conditions: H₂O(g) (steam), 300°C, phosphoric acid catalyst

What is the name of the mechanism for alkenes?

Electrophilic addition

Define electrophile

An electron pair acceptor

How do the reactions of alkenes start?

• They start via heterolytic bond fission

• This involves one atom taking both electrons from a covalent bond to form ions

What does the curly arrow show?

Movement of a pair of electrons

Electrophilic addition mechanism for ethene and HBr with explanations

1. Carbon double bond breaks + a bond is formed between a carbon and the hydrogen

2. Hydrogen to bromine bond breaks to form a Br- ion

3. Br- ion formind a bond with the carbocation intermediate

Major and minor product during electrophilic addition

• The major product is most likely to form as it forms via a more stable carbocation intermediate

• Alkyl groups act as electron donating groups which act to stabilise the carbocation

The major product forms with

• the most alkyl groups

• more carbon atoms bonded to it

Define steroisomerism

A molecule with the same structural formula but a different arrangement of atoms in space

Why does E/Z isomerism happen?

Due to the lack of rotation around the C=C

E/Z isomerism

What is cis/trans isomerism?

A type of E/Z isomerism that only happens when both carbons in the double bond have a hydrogen atom attached

Example of cis/trans isomerism

Cahn-ingold-prelog rules (CIP)

• Each atom attached to the carbonds of the double bond is assigned a priority based on atomic number

• If both higher priority group are on the zame side it is the z-isomer

• If both higher priority groups are ecross it is the E-isomer

• When the same atom is directly bonded to the carbond of the double bond you assign priority using the first point of difference

Example of cahn-ingold-prelog rules

Define polymers

Very large simple molecules formed from thousands of monomers

Define monomer

A small molecule that combines with other monomers to form a polymer

How to name polymers?

Place the prefix -poly before the name of its parent monomer

How to draw addition polymers?