Chem Spring Revision

What is the hybridisation and geometry of carbon in alkanes?

sp3 hybridised, tetrahedral geometry, bond angle 109.5°

What is free rotation in alkanes?

Rotation around C–C single (sigma) bonds

What are conformational isomers?

Different spatial arrangements due to rotation around C–C bonds

What is a Newman projection?

A way to represent rotation around a C–C bond

What is an eclipsed conformation?

Bonds aligned, highest energy due to repulsion

What is a staggered conformation?

Bonds maximally separated, lowest energy

Which conformation is most stable?

Staggered

What is torsional strain?

Repulsion between electrons in bonds on adjacent carbons

When is torsional strain highest?

In eclipsed conformations

What is steric strain?

Repulsion between bulky groups close together

Which butane conformation is most stable?

Anti

What is angle strain?

Deviation from ideal 109.5° bond angle

Why is cyclopropane unstable?

60° bond angles and eclipsing cause high strain

Why is cyclobutane not planar?

Would have eclipsed hydrogens causing strain

What shape does cyclobutane adopt?

Puckered conformation

Why does cyclopentane avoid planar structure?

To reduce eclipsing strain

What conformation does cyclopentane adopt?

Envelope

What is the most stable cyclohexane conformation?

Chair

Why is chair conformation stable?

No angle strain and no torsional strain

What happens in a chair flip?

Axial and equatorial positions swap

Which position is more stable for substituents?

Equatorial

Why is axial less stable?

1,3-diaxial interactions

Why is boat conformation unstable?

Torsional strain and steric strain

What does SN2 stand for?

Substitution nucleophilic bimolecular

What type of mechanism is SN2?

Concerted one-step

What is the SN2 rate equation?

Rate = k[Nu][R-X]

What happens in SN2?

Bond forms and breaks simultaneously

What stereochemistry occurs in SN2?

Inversion of configuration

Order of SN2 reactivity?

1° > 2° >> 3°

Why are tertiary halides slow in SN2?

Steric hindrance

Which solvents favour SN2?

Polar aprotic

Why do protic solvents slow SN2?

H-bonding stabilises nucleophile

What makes a good nucleophile?

High electron density, negative charge

What makes a good leaving group?

Weak base, stable anion

Leaving group order?

I- > Br- > Cl-

What does SN1 stand for?

Substitution nucleophilic unimolecular

How many steps in SN1?

Two

What is SN1 rate equation?

Rate = k[R-X]

What is rate determining step in SN1?

Carbocation formation

Order of carbocation stability?

3° > 2° > 1°

Why are tertiary carbocations stable?

Inductive effect and hyperconjugation

What stereochemistry occurs in SN1?

Racemisation

Which solvents favour SN1?

Polar protic

What does E2 stand for?

Elimination bimolecular

What type of mechanism is E2?

Concerted

What is removed in E2?

H and leaving group

What geometry is required for E2?

Anti-periplanar

Why is anti-periplanar required?

Correct orbital overlap

What is the product of E2?

Alkene

Which alkene is most stable?

More substituted

Why is trans product favoured?

Lower steric strain

What is the first step in electrophilic addition?

Electrophile attacks pi bond

What intermediate forms?

Carbocation

What happens next?

Nucleophile attack

What is Markovnikov’s rule?

H adds to carbon with more hydrogens

Why does this occur?

More stable carbocation forms

What rearrangements can occur?

Hydride or methyl shift

Why do rearrangements happen?

To form more stable carbocation

What intermediate forms with Br2?

Bromonium ion

What is special about benzene?

Aromatic with delocalised pi electrons

How many pi electrons in benzene?

6

What is first step in electrophilic substitution?

Electrophile attack (slow)

What intermediate forms in Electrophilic Substitution?

Arenium ion

What happens next?

Loss of H+ restores aromaticity

Why is benzene less reactive than alkenes?

Aromatic stabilisation

What do electron donating groups do?

Activate ring

Where do they direct substitution?

Ortho and para

What do electron withdrawing groups do?

Deactivate ring

Where do they direct substitution?

Meta

Why is carbonyl carbon reactive?

Polar C=O gives delta+ carbon

What type of reaction do carbonyls undergo?

Nucleophilic addition

What intermediate forms?

Tetrahedral intermediate

What does BH4- do?

Reduces carbonyl compounds

What do ketones reduce to?

Secondary alcohols

What do aldehydes reduce to?

Primary alcohols

What is a Grignard reagent?

Organomagnesium compound

What does Grignard do?

Forms C-C bonds

What is a hemiacetal?

Product of aldehyde + alcohol

What is nucleophilic acyl substitution?

Addition-elimination mechanism

What intermediate forms?

Tetrahedral intermediate

Why is Cl- a good leaving group?

Weak base and stable

Why are acyl chlorides reactive?

Strong inductive effect and good leaving group

When is substitution favoured?

Stronger nucleophile than leaving group

What are products of ester hydrolysis?

Carboxylic acid and alcohol

Why does acid increase rate?

Protonates carbonyl increasing electrophilicity

What intermediate forms in hydrolysis?

Tetrahedral intermediate

How to drive reaction forward?

Excess water

Why are amides less reactive?

Strong resonance stabilisation

What is key difference between SN1 and SN2?

SN1 two-step, SN2 one-step

Which favours tertiary substrates?

SN1

Which favours primary substrates?

SN2

SN1 stereochemistry?

Racemisation

SN2 stereochemistry?

Inversion

What determines SN2 vs E2?

Base strength and sterics

When does E2 dominate?

Strong base and hindered substrate

When does SN2 dominate?

Strong nucleophile and low sterics

What do SN1 and E1 share?

Carbocation intermediate

What determines substitution vs elimination?

Temperature and nucleophile/base

Why does benzene undergo substitution not addition?

Preserves aromaticity

Why do alkenes undergo addition?

No aromatic stabilisation