carbocations - structure

What is a reactive intermediate?

A detectable species that is potentially isolable under suitably inert conditions and is a localised energy minimum on a reaction profile.

What is a transition state?

An unobserved theoretical species at a localised energy maximum where bonds are being formed and broken.

Does an SN2 reaction contain a reactive intermediate?

No.

How many transition states does an SN1 reaction have?

Two transition states.

What reactive intermediate is formed during an SN1 reaction?

A carbocation.

What does the Hammond postulate state?

Structures close in energy that transform directly into each other are similar in structure.

Why is the highest energy transition state important?

It is usually the rate-determining step and has the greatest influence on the reaction rate.

How many valence electrons does a carbocation have?

6.

What is the shape and structure around a carbocation?

Trigonal planar,

postive charge In the empty pz orbital on the carbon atom.

Why must a carbocation adopt an sp² geometry?

It is the lowest-energy configuration for a carbon with three bonds and six valence electrons.

What three factors stabilise carbocations?

Inductive effects, resonance (mesomeric) effects and hyperconjugation.

Which is generally stronger: resonance or inductive effects?

Resonance (mesomeric) effects.

What is an inductive effect?

The ability of substituents to stabilise or destabilise adjacent groups through the sigma-bond framework.

What causes inductive effects?

Differences in electronegativity and polarisability.

What is the effect of +I groups on carbocations?

They stabilise carbocations, EDG groups

What is the effect of –I groups on carbocations?

They destabilise carbocations, EWG groups .

What is a resonance (mesomeric) effect?

Delocalisation of charge through an extended π system using adjacent p orbitals.

When can resonance stabilisation occur?

When filled and empty orbitals are adjacent and correctly aligned.

How does resonance stabilise a carbocation?

By delocalising the positive charge.

Why is an oxonium ion more stable than an equivalent alkyl carbocation?

Oxygen donates a lone pair into the empty p orbital, giving mesomeric stabilisation despite its –I effect.

What is hyperconjugation?

Stabilisation caused by overlap of a filled σ bond with an adjacent empty orbital.

Why is hyperconjugation weaker than resonance?

The orbitals only partially overlap and are not parallel.

Which bonds contribute most to hyperconjugation?

C–H and C–C sigma bonds.

How does increasing the number of alkyl groups affect hyperconjugation?

It increases hyperconjugative stabilisation.

Why is a CH₃ group better at stabilising a carbocation than a CD₃ group?

The C–H bond overlaps more effectively because the C–D bond is stronger.

What type of overlap occurs during hyperconjugation?

σ → p orbital overlap., weak psuedo pi bond formed

What is the general order of carbocation stability?

Tertiary > Secondary > Primary > Methyl.

Why are allylic carbocations stabilised?

The positive charge is delocalised by resonance over the adjacent π system.

Why are benzylic carbocations stabilised?

The positive charge is delocalised into the aromatic ring.

Why is the trityl carbocation especially stable? - draw it out

The positive charge is delocalised over three phenyl rings.

Why are aromatic carbocations stabilised?

The positive charge can be delocalised by resonance within the conjugated π system.

Why is the vinyl cation unstable?

The carbon cannot readily adopt the preferred sp² carbocation geometry with an empty p orbital.