Ch. 6 - Thermodynamics and KineticsSee all chapters
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Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

Rank the following carbocations in order of decreasing stability

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Johnny Betancourt

After we talk about what a carbocation is and the factors that affect carbocation stability, we're going to apply that to a very common type of problem that you might find on your exam: Rank the following carbocations in order of decreasing stability.

Rank the following carbocations in order of decreasing stabilityRank the following carbocations in order of decreasing stability

P.S. Check out my 24-minute-long lesson on carbocation stability if you’re not rushing!

What exactly is a carbocation?

A carbocation is a carbon atom with a positive charge. Its octet is not filled, it has an empty p-orbital, and it’s sp2-hybridized. They’re generally created when a leaving group dissociates in a substitution, elimination, or solvolysis reaction. Not all carbocations are created equal however; some of them are much more stable than others. 

Stability of carbocations:

What affects the stability of carbocations? How many other carbon atoms they’re attached to. Other R-groups will actually donate electron density to the carbocation through a process called hyperconjugation. Carbocations are electron-deficient, so the more R groups one has attached the more stable it will be! Let’s check out the trend below. 

Carbocation stabilityCarbocation stability

The more R-groups a carbocation has attached, the more stable it is! That means that tertiary is more stable than secondary, secondary more stable than primary, and primary more stable than methyl. In general, you probably won’t see a primary or methyl carbocation in O-Chem 1. 

Practice:

Let’s go ahead and sort some carbocations in terms of their stability. 

Carbocations unidentifiedCarbocations unidentified

Let's quickly identify each carbocation as methyl, primary, and so on.

Carbocations identifiedCarbocations identified

Now that we know what kinds of carbocation each one is, it should be really easy to place them in the right order! 

Carbocations sortedCarbocations sorted

Tertiary is on top since it’s the most stable due to its R-groups, and methyl is on bottom because it has no R-groups.

Okay, cool! We’ve sorted carbocations in order of decreasing stability! But what happens if a carbocation is allylic, i.e. adjacent to a double bond? My videos on carbocation stability go over that and more! Good luck studying!



Johnny Betancourt

Johnny got his start tutoring Organic in 2006 when he was a Teaching Assistant. He graduated in Chemistry from FIU and finished up his UF Doctor of Pharmacy last year. He now enjoys helping thousands of students crush mechanisms, while moonlighting as a clinical pharmacist on weekends.