Organic Chemistry Practice Problems and Problem Sets
SN1 mechanism
Rank the following compounds in order of decreasing reactivity with water (solvolysis). (1 = most reactive)
This is an SN1 reaction. We know this because none of the reagents have charges (H2O is neutral; if it were HO-, it would probably be SN2 or E2). So a carbocation will be formed in this reaction, and the compound whose carbocation is the most stable will react the fastest. So the 3º bromide will react fastest with water.
MendelSet practice problem # 542 submitted by Matt on July 3, 2011.
Let's work through a 1,2 and 1,4 addition. Draw the structures for each of the species in the six boxes below. Also draw curved arrows to show electron movement.
This is an SN1 reaction, but with a twist.
The top reaction is a regular SN1 reaction; the leaving group (Br-) leaves and the nucleophile (CH3OH) attacks the carbocation.
The twist is that the carbocation is allylic, and has resonance, so there is another carbon that the nucleophile can attack.
So instead of just one SN1 product, two products are formed. The product that doesn't involve resonance is called the direct addition or 1,2 product. The product that involves allylic resonance is called the conjugate addition or 1,4 product.
MendelSet practice problem # 522 submitted by Matt on July 1, 2011.
For the reaction below, draw the structures of the carbocation intermediate and the final product.
This is a substitution reaction. Because the reaction takes place in acid and the leaving group is on a 2º carbon, it will probably form a carbocation (SN1 mechanism).
-OH is a poor leaving group so the alcohol will protonate first, so it can leave as H2O, which is neutral.
MendelSet practice problem # 347 submitted by Matt on June 7, 2011.
Predict the product(s) of the reaction below, and used curved arrows to show a mechanism.
The 2° carbocation formed immediately undergoes a 1,2-methyl shift (a rearrangement) to form the more stable 3° carbocation, so the product is the 3° alkyl chloride instead of the 2º alkyl chloride, which would have formed in the absence of rearrangement.
MendelSet practice problem # 337 submitted by Matt on June 7, 2011.
Carbocations aren't very stable and so don't last very long after they are formed.
Use curved arrows to show:
a) how a carbocation reacts with a halide ions to form an alkyl halide.
b) how a carbocation reacts with water to form an alcohol.
c) how a carbocation reacts with a base to form an alkene.
For a), the product is neutral and so you are done after the nucleophile (Cl-) attacks the carbocation.
For b), the the intermediate is a protonated alcohol, and so you must do a proton exchange step (also called a hydrogen exchange or deprotonation) to get the final alcohol product, which is neutral.
For c), there are two different types of beta hydrogens, and so two different alkenes are possible.
The more stable alkene is the one that will form, and this will always be the most highly substituted alkene. This is Zaitsev's rule. The rationale for this is hyperconjugation: neighboring carbon atoms stabilize an alkene.
MendelSet practice problem # 335 submitted by Matt on June 7, 2011.