The Mechanisms of Addition Reactions

Alkenes

What is an electrophile?

A species that is attracted to a region of high electron density.

Why are alkenes attractive to electrophiles?

The C=C double bond contains a pi bond with a region of high electron density.

Where is the region of high electron density in an alkene?

Around the C=C double bond due to the electrons in the pi bond.

What type of arrows are used in electrophilic addition mechanisms?

Full curly arrows.

What do full curly arrows represent?

The movement of a pair of electrons.

Where must a curly arrow start from?

A bond or a lone pair of electrons.

Where must a curly arrow point to?

An atom where the pair of electrons is moving.

Why is hydrogen bromide a polar molecule?

Bromine is more electronegative than hydrogen, so Br is δ− and H is δ+.

What are the partial charges in HBr?

H is δ+ and Br is δ−.

Why does HBr act as an electrophile when reacting with an alkene?

The δ+ hydrogen is attracted to the electron-rich pi bond of the alkene.

What is electrophilic addition?

An addition reaction in which two molecules form one molecule and the attacking species is an electrophile.

What type of reaction occurs between an alkene and a hydrogen halide?

Electrophilic addition.

What type of bond fission occurs in HBr during electrophilic addition?

Heterolytic fission.

What is heterolytic fission?

The breaking of a covalent bond so that both bonding electrons are taken by one atom.

Why do both electrons from the H–Br bond go to bromine?

Bromine is more electronegative than hydrogen.

What ions are formed when the H–Br bond breaks during electrophilic addition?

A carbocation and a bromide ion, Br⁻.

What is a carbocation?

A positive ion in which the positive charge is on a carbon atom.

What happens in Step 1 of the electrophilic addition of HBr to ethene?

The pi electrons attack the δ+ hydrogen of HBr, the H–Br bond breaks heterolytically, and a carbocation and Br⁻ ion form.

What happens to the pi electrons in Step 1 of the reaction between ethene and HBr?

They form a new covalent bond between a carbon atom and hydrogen.

What happens to the electrons in the H–Br bond during Step 1?

Both electrons move to bromine, producing Br⁻.

What happens in Step 2 of the electrophilic addition of HBr to ethene?

Br⁻ donates a lone pair to the positively charged carbon of the carbocation, forming a C–Br bond.

What is the final product when ethene reacts with HBr?

Bromoethane.

What is the overall equation for the reaction between ethene and HBr?

CH₂=CH₂ + HBr → CH₃CH₂Br.

Why are oppositely charged ions attracted during Step 2?

The negatively charged bromide ion is attracted to the positively charged carbocation.

What type of reaction occurs between an alkene and a halogen such as bromine?

Electrophilic addition.

Why can bromine react by electrophilic addition even though Br₂ is non-polar?

The electron-rich pi bond repels electrons in Br₂ and induces a temporary dipole.

What happens to Br₂ as it approaches the C=C bond?

It becomes polarised, producing a δ+ bromine atom and a δ− bromine atom.

Which part of polarised Br₂ is attacked by the alkene pi bond?

The δ+ bromine atom.

What happens in Step 1 of the electrophilic addition of Br₂ to ethene?

The pi electrons attack Brδ+, the Br–Br bond breaks heterolytically, and a carbocation and Br⁻ form.

What happens in Step 2 of the electrophilic addition of Br₂ to ethene?

Br⁻ donates a lone pair to the carbocation, forming the second C–Br bond.

What product forms when ethene reacts with bromine?

1,2-dibromoethane.

What is the overall equation for ethene reacting with bromine?

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br.

What is an asymmetrical alkene?

An alkene in which the atoms or groups attached to the two carbon atoms of the C=C bond are not the same.

What is an asymmetrical attacking molecule?

A molecule in which the atoms at its two ends are different.

What can happen when both the alkene and attacking molecule are asymmetrical?

Two different products may be formed.

Why can two products form when an asymmetrical molecule reacts with an asymmetrical alkene?

The attacking molecule can add across the C=C bond in two different orientations.

What products can form when propene reacts with HBr?

2-bromopropane and 1-bromopropane.

What is the major product when propene reacts with HBr?

2-bromopropane.

What is the minor product when propene reacts with HBr?

1-bromopropane.

Why are unequal amounts of products formed when propene reacts with HBr?

The two possible carbocation intermediates have different stabilities.

What determines which product is the major product in electrophilic addition?

The major product forms through the more stable carbocation intermediate.

What is a primary carbocation?

A carbocation where the positively charged carbon is attached to one alkyl group.

What is a secondary carbocation?

A carbocation where the positively charged carbon is attached to two alkyl groups.

What is a tertiary carbocation?

A carbocation where the positively charged carbon is attached to three alkyl groups.

What is the order of carbocation stability?

Tertiary > secondary > primary.

Why are tertiary carbocations more stable than secondary carbocations?

They have more electron-releasing alkyl groups attached to the positively charged carbon.

Why are secondary carbocations more stable than primary carbocations?

They have more electron-releasing alkyl groups that help spread the positive charge.

What is an electron-releasing group?

A group that pushes electrons towards the atom to which it is joined.

How do alkyl groups stabilise carbocations?

They release electron density towards the positively charged carbon, spreading the positive charge.

Which carbocation is formed preferentially when propene reacts with HBr?

The secondary carbocation.

Why is the secondary carbocation formed from propene more stable than the primary carbocation?

Its positively charged carbon is attached to two electron-releasing alkyl groups rather than one.

Why is 2-bromopropane the major product when propene reacts with HBr?

It forms through the more stable secondary carbocation.

Why is 1-bromopropane the minor product when propene reacts with HBr?

It forms through the less stable primary carbocation.

What is the relationship between carbocation stability and major product formation?

The more stable the carbocation intermediate, the more likely it is to form and therefore the greater the amount of its product.

What is the stability order of primary, secondary and tertiary carbocations?

Tertiary carbocations are most stable, followed by secondary, then primary.

What type of reaction occurs when alkanes react with halogens?

Free radical substitution.

What type of reaction occurs when alkenes react with halogens?

Electrophilic addition.

What is the key difference between homolytic and heterolytic fission?

Homolytic fission gives one bonding electron to each atom, whereas heterolytic fission gives both bonding electrons to one atom.