Chem 351 Fall 2011 MT Mechanism

Part 7: MECHANISM

The following diagram shows the solution to the mechanistic question. Note that all the information applies to a single reaction sequence that has been completely described verbally. There is no need for extra reagents or extra steps etc. The curly arrows are drawn specifically to match the text in the question. The biggest problem students have is making sure they understand the language of chemistry. Most students have trouble because they can't draw the structures from the IUPAC names (that means they don't know their nomenclature well enough). Read the words carefully, and then make the curly arrows tell that same story. There is NO need for extra steps. Remember curly arrows go from electron rich to poor and to balance the formal charges at each step - errors on formal charges were common.

a.

mechanism

If you struggled with this part of the question, first draw the compounds whose names were provided, then think about the types of reactions (e.g. acid / base) and try to fill in the structures in the gaps, then finally add the required curly arrows to account for all the bonding changes.

b. Since the reaction requires deprotonation of a terminal alkyne (pKa about 25) we need quite a strong base (i.e. pKa > 25). A reasonable example would be a alkyl carbanion such as CH₃^- (pKa > 50). Note that the amide ion (NH₂^-) used in the paragraph has a pKa about 35.

hex-2-yne

2-methylhex-3-yne

e. In the analogous reaction of but-1-ene with ethyl iodide, sodium amide (pKa = 35) is not a strong enough base to remove a proton from an sp2 C-H bond in the alkene (pKa = 45), thus one can not prepare the required nucleophile for the reaction to work unless you were to use a stronger base.

Common errors:

could not draw the structure of ethyl idodide correctly (e.g. drawing CH₃CH₂CH₂I instead)
could not draw sodium amide (NaNH₂) and / or ammonia (NH₃) correctly. The "tough" one here was the "amide ion", but if you drew ammonia and then reversed the reaction and removed a proton, it would have "told" you the amide ion.
not removing the correct H (i.e. most acidic H is from the terminal alkyne, C≡C-H pKa = 25)
drawing structures that break valence rules
incorrect formal charges (extra or missing) (including drawing a carbocation instead of the carbanion as stated in the question)
incorrect, vague, backwards and incomplete curly arrows. Drawing radical arrows (read the question !). Arrows are required to account for all the bonding changes.
showing extra steps not described in the question (so in effect answering a different question)
using single headed arrows (radical) despite the question saying to use double headed arrows

b. Poor knowledge of acids and bases e.g. suggesting things like HO- , RO- , H₂O etc or worst , H₂SO₄

c & d. Did not use part a as the basis of working out the answer. Only showing one component, instead of simply drawing a structure trying to name a compound and not doing it correctly (i.e. poor nomenclature). Not taking into account the number of C atoms that were being combined. c. Suggesting but-2-yne but there is no terminal alkyne C-H to be removed there. Suggesting but-3-yne (incorrect naming).

d. Suggesting 2-methylbut-1-yne.... an impossible structure

e. Poor knowledge of acidity. Rationale based on pKa alone, incomplete rationale. Answering a different question because they did not read the question asked carefully enough and therefore made avoidable errors including talking about the triple bond in a butene (that's an alkene). Talking about radical stability or C-H bond strengths... since the reaction does not involve radicals, they are irrelevant. Bond strength is not a good predictor of acidity.