3 Factors That Stabilize Carbocations

8/13/2019 3 Factors That Stabilize Carbocations

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3 Factors That StabilizeCarbocationsinDRAWING REACTION MECHANISMS,ORGANIC CHEMISTRY 1,WHERE

ELECTRONS ARE

If electrons were money, carbocations would be thebeggars of organic chemistry. Packing a mere six valenceelectrons, these electron-deficient intermediates figureprominently in many reactions we meet in organicchemistry, such as

nucleophilic substitution (SN1) and elimination (E1)reactions

additions of electrophiles to double and triple bonds ,electrophilic aromatic substitution additions to carbonyl compounds and enolate chemistry

(albeit in masked form)

Thats a huge chunk of sophomore O-chem, right there.

Being electron-deficient (and therefore unstable),formation of a carbocation is usually the rate-limiting stepin these reactions.

Knowing that, then think about this: what happens to therate of the reaction when the carbocation intermediate is

made more stable?Well, the energy of the transition stateleading to the reaction will be lower.Whats that going to do to the rate of the reaction?Itsgoing to speed it up.So what are some of the factors that stabilizecarbocations?
http://masterorganicchemistry.com/category/drawing-reaction-mechanisms/http://masterorganicchemistry.com/category/drawing-reaction-mechanisms/http://masterorganicchemistry.com/category/organic-chemistry-1/http://masterorganicchemistry.com/category/where-electrons-are/http://masterorganicchemistry.com/category/where-electrons-are/http://masterorganicchemistry.com/category/where-electrons-are/http://masterorganicchemistry.com/category/where-electrons-are/http://masterorganicchemistry.com/category/where-electrons-are/http://masterorganicchemistry.com/category/organic-chemistry-1/http://masterorganicchemistry.com/category/drawing-reaction-mechanisms/


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If you look through all of your organic chemistry textbook,youll find 3 main structural factors that help to stabilizecarbocations.

1. Neighboring carbon atoms.2. Neighboring carbon-carbon multiple bonds3. Neighboring atoms with lone pairs.Why is this? It all goes back to the core governing force inchemistry: electrostatics. Since opposite charges attract,like charges repel, you would be right in thinkingthat carbocations are stabilized by nearby electron-

donating groups.Lets look at each of these in turn.

1) Carbocations are stabilized by neighboring carbonatoms.The stability of carbocations increases as we go fromprimary to secondary to tertiary carbons. Theres two

answers as to why this is. The age-old answer that is stillpassed around in many introductory textbooks points tocarbons (alkyl groups in particular) as being electron-releasing groups through inductive effects. That is, acarbon (electronegativity 2.5) connected to hydrogen(electronegativity 2.2) will be electron rich, and candonate some of those electrons to the neighboringcarbocation. In other words, the neighboring carbon

pays the carbocation with electrons it steals from thehydrogens. The second, (and theoretically moresatisfactory explanation) ishyperconjugation,whichinvokes stabilization through donation of the electrons inC-H sigma bonds to the empty p orbital of thecarbocation.Whatever the explanation, this factor governs many key

reactions you meet in Org 1fromMarkovnikoffs rule,
http://en.wikipedia.org/wiki/Hyperconjugationhttp://en.wikipedia.org/wiki/Hyperconjugationhttp://en.wikipedia.org/wiki/Hyperconjugationhttp://en.wikipedia.org/wiki/Markovnikovhttp://en.wikipedia.org/wiki/Markovnikovhttp://en.wikipedia.org/wiki/Markovnikovhttp://en.wikipedia.org/wiki/Hyperconjugation


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tocarbocation rearrangements,through understandingthe SN1 and E1 reactions.

2) Carbocations are stabilized by neighboring carbon-carbon multiple bonds. Carbocations adjacent toanother carbon-carbon double or triple bond have special

stability because overlap between the empty p orbital ofthe carbocation with the p orbitals of the bond allows forcharge to be shared between multiple atoms. This effect,called delocalization is illustrated by drawing resonance

structures where the charge moves from atom to atom.This is such a stabilizing influence that even primarycarbocationsnormally very unstableare remarkably

easy to form when adjacent to a double bond, so much sothat they will actually participate in SN1 reactions.
http://en.wikipedia.org/wiki/Wagner%E2%80%93Meerwein_rearrangementhttp://en.wikipedia.org/wiki/Wagner%E2%80%93Meerwein_rearrangementhttp://en.wikipedia.org/wiki/Wagner%E2%80%93Meerwein_rearrangementhttp://en.wikipedia.org/wiki/Delocalized_electronhttp://masterorganicchemistry.files.wordpress.com/2011/03/1-substitution-copy.jpghttp://en.wikipedia.org/wiki/Delocalized_electronhttp://en.wikipedia.org/wiki/Wagner%E2%80%93Meerwein_rearrangement


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3) Carbocations are stabilized by adjacent lonepairs. The key stabilizing influence is a neighboring atomthat donates a pair of electrons to the electron-poor

carbocation. Note here that this invariably results informing a double bond ( bond) and the charge willmove to the atom donating the electron pair. Hence thisoften goes by the name of donation.The strength of this effect varies with basicity, so nitrogenand oxygen are the most powerful donors. Strangelyenough, even halogens can help to stabilize carbocations

through donation of a lone pair. The fact that atoms thatwe normally think of as electron-wthdrawing(nitrogen, oxygen, chlorine) can actually be electron-donor groups is probably one of the most difficultfactors to wrap your head around in Org 2.This effect is tremendously important in the reactions ofaromatic rings and also in enolate chemistry, wheredouble bonds attached to donating groups (nitrogen and
http://masterorganicchemistry.files.wordpress.com/2011/03/2-adjacent-double-bonds-copy.jpg


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oxygen in particular) can be millions (or billions)of timesmore nucleophilic than alkenes that lack these groups.

The bottom line of this post is that by understanding thefactors which affect the stability of carbocations, youcan gain tremendous insight into many differentreactions, even though they may appear vastlydifferent.

Why is this important? Many reactions pass throughcarbocation intermediates. What do you think the effect ofstabilizing the carbocation will be on the reaction rates?Heres some specific examples.
http://masterorganicchemistry.files.wordpress.com/2011/03/reactscalesposter.pdfhttp://masterorganicchemistry.files.wordpress.com/2011/03/reactscalesposter.pdfhttp://masterorganicchemistry.files.wordpress.com/2011/03/3-lone-pairs-copy.jpghttp://masterorganicchemistry.files.wordpress.com/2011/03/reactscalesposter.pdf


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Carbocations are stabilized by neighboring carbon atoms.

See few examples below:
http://masterorganicchemistry.files.wordpress.com/2011/03/4-apply-concept-copy.jpghttp://masterorganicchemistry.files.wordpress.com/2011/03/4-apply-concept-copy1.jpghttp://masterorganicchemistry.files.wordpress.com/2011/03/4-apply-concept-copy.jpg


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Carbocations are stabilized by neighboring carbon-carbon multiple bonds

Examples below:

Carbocations are stabilized by adjacent lone pairs

Examples below


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In a long chain or cyclic compound additional +I and +R effects furtherstabilize the Carbocation

3 Factors That StabilizeCarbocationsinDRAWING REACTION MECHANISMS,ORGANIC CHEMISTRY 1,WHERE

ELECTRONS ARE
http://masterorganicchemistry.com/category/drawing-reaction-mechanisms/http://masterorganicchemistry.com/category/drawing-reaction-mechanisms/http://masterorganicchemistry.com/category/organic-chemistry-1/http://masterorganicchemistry.com/category/where-electrons-are/http://masterorganicchemistry.com/category/where-electrons-are/http://masterorganicchemistry.com/category/where-electrons-are/http://masterorganicchemistry.com/category/where-electrons-are/http://masterorganicchemistry.com/category/where-electrons-are/http://masterorganicchemistry.com/category/organic-chemistry-1/http://masterorganicchemistry.com/category/drawing-reaction-mechanisms/


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If electrons were money, carbocations would be thebeggars of organic chemistry. Packing a mere six valenceelectrons, these electron-deficient intermediates figure

prominently in many reactions we meet in organicchemistry, such as

nucleophilic substitution (SN1) and elimination (E1)reactions

additions of electrophiles to double and triple bonds ,electrophilic aromatic substitution additions to carbonyl compounds and enolate chemistry

(albeit in masked form)Thats a huge chunk of sophomore O-chem, right there.

Being electron-deficient (and therefore unstable),formation of a carbocation is usually the rate-limiting stepin these reactions.

Knowing that, then think about this: what happens to therate of the reaction when the carbocation intermediate ismade more stable?Well, the energy of the transition stateleading to the reaction will be lower.Whats that going to do to the rate of the reaction?Itsgoing to speed it up.So what are some of the factors that stabilizecarbocations?

If you look through all of your organic chemistry textbook,youll find 3 main structural factors that help to stabilizecarbocations.

1. Neighboring carbon atoms.2. Neighboring carbon-carbon multiple bonds

3. Neighboring atoms with lone pairs.


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Why is this? It all goes back to the core governing force inchemistry: electrostatics. Since opposite charges attract,like charges repel, you would be right in thinking

that carbocations are stabilized by nearby electron-donating groups.Lets look at each of these in turn.

1) Carbocations are stabilized by neighboring carbonatoms.The stability of carbocations increases as we go fromprimary to secondary to tertiary carbons. Theres two

answers as to why this is. The age-old answer that is stillpassed around in many introductory textbooks points tocarbons (alkyl groups in particular) as being electron-releasing groups through inductive effects. That is, acarbon (electronegativity 2.5) connected to hydrogen(electronegativity 2.2) will be electron rich, and candonate some of those electrons to the neighboring

carbocation. In other words, the neighboring carbonpays the carbocation with electrons it steals from thehydrogens. The second, (and theoretically moresatisfactory explanation) ishyperconjugation,whichinvokes stabilization through donation of the electrons inC-H sigma bonds to the empty p orbital of thecarbocation.Whatever the explanation, this factor governs many keyreactions you meet in Org 1fromMarkovnikoffs rule,tocarbocation rearrangements,through understandingthe SN1 and E1 reactions.
http://en.wikipedia.org/wiki/Hyperconjugationhttp://en.wikipedia.org/wiki/Hyperconjugationhttp://en.wikipedia.org/wiki/Hyperconjugationhttp://en.wikipedia.org/wiki/Markovnikovhttp://en.wikipedia.org/wiki/Markovnikovhttp://en.wikipedia.org/wiki/Wagner%E2%80%93Meerwein_rearrangementhttp://en.wikipedia.org/wiki/Wagner%E2%80%93Meerwein_rearrangementhttp://en.wikipedia.org/wiki/Wagner%E2%80%93Meerwein_rearrangementhttp://masterorganicchemistry.files.wordpress.com/2011/03/1-substitution-copy.jpghttp://en.wikipedia.org/wiki/Wagner%E2%80%93Meerwein_rearrangementhttp://en.wikipedia.org/wiki/Markovnikovhttp://en.wikipedia.org/wiki/Hyperconjugation


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2) Carbocations are stabilized by neighboring carbon-carbon multiple bonds. Carbocations adjacent toanother carbon-carbon double or triple bond have special

stability because overlap between the empty p orbital ofthe carbocation with the p orbitals of the bond allows forcharge to be shared between multiple atoms. This effect,called delocalization is illustrated by drawing resonance

structures where the charge moves from atom to atom.This is such a stabilizing influence that even primarycarbocationsnormally very unstableare remarkablyeasy to form when adjacent to a double bond, so much sothat they will actually participate in SN1 reactions.

3) Carbocations are stabilized by adjacent lonepairs. The key stabilizing influence is a neighboring atomthat donates a pair of electrons to the electron-poorcarbocation. Note here that this invariably results informing a double bond ( bond) and the charge willmove to the atom donating the electron pair. Hence this

often goes by the name of donation.
http://en.wikipedia.org/wiki/Delocalized_electronhttp://masterorganicchemistry.files.wordpress.com/2011/03/2-adjacent-double-bonds-copy.jpghttp://en.wikipedia.org/wiki/Delocalized_electron


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The strength of this effect varies with basicity, so nitrogenand oxygen are the most powerful donors. Strangelyenough, even halogens can help to stabilize carbocations

through donation of a lone pair. The fact that atoms thatwe normally think of as electron-wthdrawing(nitrogen, oxygen, chlorine) can actually be electron-donor groups is probably one of the most difficultfactors to wrap your head around in Org 2.This effect is tremendously important in the reactions ofaromatic rings and also in enolate chemistry, wheredouble bonds attached to donating groups (nitrogen andoxygen in particular) can be millions (or billions)of timesmore nucleophilic than alkenes that lack these groups.

The bottom line of this post is that by understanding thefactors which affect the stability of carbocations, youcan gain tremendous insight into many different
http://masterorganicchemistry.files.wordpress.com/2011/03/reactscalesposter.pdfhttp://masterorganicchemistry.files.wordpress.com/2011/03/reactscalesposter.pdfhttp://masterorganicchemistry.files.wordpress.com/2011/03/3-lone-pairs-copy.jpghttp://masterorganicchemistry.files.wordpress.com/2011/03/reactscalesposter.pdf


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reactions, even though they may appear vastlydifferent.Why is this important? Many reactions pass through

carbocation intermediates. What do you think the effect ofstabilizing the carbocation will be on the reaction rates?Heres some specific examples.
http://masterorganicchemistry.files.wordpress.com/2011/03/4-apply-concept-copy1.jpghttp://masterorganicchemistry.files.wordpress.com/2011/03/4-apply-concept-copy.jpg

3 Factors That Stabilize Carbocations

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