Reaction Points CH:CH2CH2OH H2SO4 Please select file(s) Select file(s) 02.2 Reaction B Points OH HI Please select file(s) Select file(s) 02.3 Reaction C Points H2SO4 MCPBA OH 180°C CH2Cl2 Please select file(s) Select file(s) 02.4 Reaction D Points H2CrO4 (excess) HO- OH Please select file(s) Select file(s) 02.5 Reaction E Points HBr (1 eq) PCC CH2Cl2 Please select file(s) Select file(s)
The Correct Answer and Explanation is:
Here are the products for each reaction, followed by a detailed explanation.
Correct Answers:
- Q2.1 Reaction A Product: The product is (1S,2R,3R,5R)-5-butoxy-1,3-dimethylcyclopentan-2-ol. The butoxy group is attached to the more substituted carbon of the original epoxide with inverted stereochemistry (trans to the hydroxyl group).
- Q2.2 Reaction B Product: The product is 4-iodo-4-methylheptane.
- Q2.3 Reaction C Product: The final product is 2,2-dimethyloxirane.
- Q2.4 Reaction D Product: The product is (S)-4-hydroxy-4-methylheptanoic acid.
- Q2.5 Reaction E Product: The final product is pentanedial (also known as glutaraldehyde), OHC-(CH2)3-CHO.
Explanation:
Reaction A is an acid-catalyzed ring-opening of an epoxide. The sulfuric acid (H2SO4) protonates the epoxide oxygen, making it a better leaving group. The alcohol, butan-1-ol, then acts as a nucleophile. In acidic conditions, the nucleophile attacks the more substituted carbon of the epoxide because the transition state has significant carbocation character, which is stabilized by hyperconjugation. The attack occurs with inversion of stereochemistry (SN2-like), resulting in a trans-diol ether.
Reaction B involves the reaction of a secondary alcohol with hydroiodic acid (HI). This proceeds through an SN1 mechanism. The alcohol is first protonated by the strong acid to form water, a good leaving group. Departure of water generates a secondary carbocation. This carbocation undergoes a rapid 1,2-hydride shift from the adjacent carbon to form a more stable tertiary carbocation. The iodide ion (I-) then attacks this stable carbocation to yield the final alkyl iodide product.
Reaction C is a two-step synthesis. First, the primary alcohol undergoes acid-catalyzed dehydration at high temperature. This E1 reaction involves protonation, loss of water to form an unstable primary carbocation, which immediately rearranges via a 1,2-hydride shift to a stable tertiary carbocation. Elimination of a proton then forms the alkene, 2-methylpropene. In the second step, this alkene is treated with mCPBA, a peroxyacid that delivers an oxygen atom to the double bond in a concerted reaction, forming an epoxide.
Reaction D is an oxidation reaction. The starting material has both a primary and a tertiary alcohol. Chromic acid (H2CrO4) is a strong oxidizing agent. It will oxidize the primary alcohol at the end of the chain all the way to a carboxylic acid. Tertiary alcohols cannot be oxidized under these conditions because they lack a hydrogen atom on the carbon bearing the hydroxyl group. Thus, the tertiary alcohol remains unchanged.
Reaction E is another two-step process. First, the starting material, an acetal, is cleaved by HBr. The reaction proceeds via an SN2 mechanism where the bromide ion attacks the least sterically hindered carbon of the ether linkages, which is the primary carbon of the propyl group. This cleavage yields 1-bromopropane and 5-hydroxypentanal. In the second step, 5-hydroxypentanal is treated with PCC, a mild oxidizing agent. PCC oxidizes the primary alcohol group to an aldehyde. It is not strong enough to oxidize the existing aldehyde to a carboxylic acid, so the final product is the dialdehyde, pentanedial.
