3 hexanone from 3 hexyne with reaction
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3. Synthesize 3-hexanone using acetylene as your only source of carbon atoms:
Synthetically we have two problems: 1) how to we take a two carbon starting material to six carbons; and 2) how do we synthezise a ketone? At this point we have seen two different ways of making ketones: 1) permanganate oxidation of an alkene with no hydrogens attached to the sp2 carbon; and 2) the hydration of an acetylene. The first route involves cleavage of the carbon-carbon pi bond which means we have to make a molecule which contains more than six atoms. The second route is just an addition reaction which means its precursor would have only six atoms. The second route, at first glance looks easier and probably would give more product. There are two alkynes, 2-hexyne and 3-hexyne, which can give 3-hexanone.
We remember that the hydration of an acetylene follows Markovnikov's Rule. However, internal (nonterminal) alkynes can give mixtures of two products since the sp carbons of all internal alkynes can only be bonded to one other carbon. In essence, Markovnikov's Rule only works for terminal alkynes.
Consider the hydration of 2-hexyne. The oxygen can end up on carbon 2 or carbon 3 giving both 2-hexanone and 3 hexanone.
Consider the hydration of 3-hexyne. The oxygen can end up on carbon 3 or carbon 4.
Note that these "two products" are identical!! The hydration of a symmetric alkyne works since only one product is formed while the hydration of a nonsymmetrical alkyne fails since it gives two
Synthetically we have two problems: 1) how to we take a two carbon starting material to six carbons; and 2) how do we synthezise a ketone? At this point we have seen two different ways of making ketones: 1) permanganate oxidation of an alkene with no hydrogens attached to the sp2 carbon; and 2) the hydration of an acetylene. The first route involves cleavage of the carbon-carbon pi bond which means we have to make a molecule which contains more than six atoms. The second route is just an addition reaction which means its precursor would have only six atoms. The second route, at first glance looks easier and probably would give more product. There are two alkynes, 2-hexyne and 3-hexyne, which can give 3-hexanone.
We remember that the hydration of an acetylene follows Markovnikov's Rule. However, internal (nonterminal) alkynes can give mixtures of two products since the sp carbons of all internal alkynes can only be bonded to one other carbon. In essence, Markovnikov's Rule only works for terminal alkynes.
Consider the hydration of 2-hexyne. The oxygen can end up on carbon 2 or carbon 3 giving both 2-hexanone and 3 hexanone.
Consider the hydration of 3-hexyne. The oxygen can end up on carbon 3 or carbon 4.
Note that these "two products" are identical!! The hydration of a symmetric alkyne works since only one product is formed while the hydration of a nonsymmetrical alkyne fails since it gives two
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