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A carbocation is stabilized by anything that feeds its empty p orbital: neighbouring C–H bonds by hyperconjugation, so tertiary beats secondary beats primary, and far more a neighbouring pi bond by resonance, as in allylic and benzylic cations; cations shift a hydride to become more stable.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will rank alkyl cations, count hyperconjugating bonds, name what stabilizes each cation, predict a hydride shift and its product, and explain why a benzylic chloride reacts fast by SN1.
You have used 'tertiary cations are more stable' to explain SN1, E1 and Markovnikov addition. Here is the reason, and the cases where resonance beats substitution.
A carbocation is a carbon with only three bonds and a positive charge; it is sp2, with an empty p orbital. Hyperconjugation is overlap of a neighbouring C–H or C–C bond with that empty orbital. Allylic and benzylic cations sit next to a C=C or a benzene ring. A hydride shift moves a hydrogen, with its pair, to a neighbouring cationic carbon.
A carbocation's positive carbon has an empty p orbital. Anything nearby that can share electron density with it spreads the charge and stabilizes the cation.
Hyperconjugation. Each C–H or C–C bond on a neighbouring carbon can lean its electrons into the empty orbital. The tert-butyl cation has nine such C–H bonds; the ethyl cation three; the methyl cation none. Hence
tertiary > secondary > primary > methyl.
Resonance. A neighbouring C=C or benzene ring does far more, spreading the charge onto other carbons. The allyl cation, CH2=CH–CH2+, has two equivalent contributors; the benzyl cation spreads its charge round the ring. Primary by count, both are about as stable as a secondary or tertiary alkyl cation.
Another way: steps
To judge a cation:
A carbocation lasts long enough to improve itself. If a hydrogen on the next carbon can move over with its bonding pair — a hydride shift — and leave a more stable cation behind, it does so before the nucleophile arrives. HBr with 3-methylbut-1-ene first gives a secondary cation next to a tertiary C–H; the hydride shifts, the charge moves to the tertiary carbon, and the product is 2-bromo-2-methylbutane. Methyl groups can shift the same way. Whenever a reaction goes through a carbocation, look one carbon away for a better one.
A primary carbocation is always more stable than a tertiary one. The reverse, for alkyl cations.
Carbocations never rearrange. They shift H or CH3 to become more stable.
Every primary cation is unstable. Allylic and benzylic ones are stabilized by resonance.
Hyperconjugation counts hydrogens on the positive carbon. It counts those on its neighbours.
Its positive carbon is bonded to a CH3 and a CH2.
Two carbons: secondary.
C–H bonds next door: 3 on the CH3 and 2 on the CH2, 5 in all.
Count the neighbours' hydrogens.
So it is more stable than the ethyl cation but less than tert-butyl.
Between primary and tertiary.
Which has a neighbouring pi bond?
The allyl cation.
Which is more stable, and why?
The allyl cation: resonance spreads its charge over two carbons.
Rank these carbocations from most stable to least: ethyl CH3CH2+, methyl CH3+, tert-butyl (CH3)3C+, isopropyl (CH3)2CH+.
Number the steps in order (write the number in the box):
How many C–H bonds are on the carbons next to the positive carbon of the methyl cation, CH3+, able to feed it electron density by hyperconjugation?
Answer:
Match each carbocation to what stabilizes its charge.
| alkyl groups, by hyperconjugation | resonance with a neighbouring C=C | resonance with a benzene ring | nothing: the least stable | |
|---|---|---|---|---|
| the tert-butyl cation, (CH3)3C+ | ||||
| the allyl cation, CH2=CH–CH2+ | ||||
| the benzyl cation, C6H5CH2+ | ||||
| the methyl cation, CH3+ |
HBr adds to 3-methylbut-1-ene, CH2=CH–CH(CH3)2. The first cation forms on carbon 2, next to a carbon carrying one H and two methyls. What happens next, and what is the major product?
In warm aqueous ethanol, benzyl chloride, C6H5CH2Cl, reacts by SN1 thousands of times faster than chloroethane, CH3CH2Cl, though both are primary chlorides. Why?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
For each carbocation, give its class and whether resonance spreads its charge.
| class | resonance-stabilized | |
|---|---|---|
| (CH3)2CH+ | ||
| CH2=CH–CH2+ | ||
| (CH3)3C+ |
You can say why one carbocation is more stable than another. Tell someone why benzyl chloride reacts by SN1 though it is primary. Next: the stereochemistry of additions.
8. Your turn: the allyl cation against the propyl cation, CH3CH2CH2+, step 3