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Phosgene is highly toxic and was used as a chemical weapon
Chapter 21, Problem 21.61(choose chapter or problem)
Phosgene is highly toxic and was used as a chemical weapon in World War I. It is also a synthetic precursor used in the production of many plastics. (a) When vapors of phosgene are inhaled, the compound rapidly reacts with any nucleophilic sites present (OH groups, NH2 groups, etc.), producing HCl gas. Draw a mechanism for this process. (b) When phosgene is treated with ethylene glycol (HOCH2CH2OH), a compound with molecular formula C3H4O3 is obtained. Draw the structure of this product. (c) Predict the product that is expected when phosgene is treated with excess phenylmagnesium bromide, followed by water.
Questions & Answers
QUESTION:
Phosgene is highly toxic and was used as a chemical weapon in World War I. It is also a synthetic precursor used in the production of many plastics. (a) When vapors of phosgene are inhaled, the compound rapidly reacts with any nucleophilic sites present (OH groups, NH2 groups, etc.), producing HCl gas. Draw a mechanism for this process. (b) When phosgene is treated with ethylene glycol (HOCH2CH2OH), a compound with molecular formula C3H4O3 is obtained. Draw the structure of this product. (c) Predict the product that is expected when phosgene is treated with excess phenylmagnesium bromide, followed by water.
ANSWER:Step 1 of 3
a)
First, the molecule reacts with any nearby nucleophiles to produce HCl gas. Draw a
mechanism for this process.
The most carbonyl carbons are quite electrophilic, due to a combination of inductive and
resonance effects. This makes the carbonyl carbon a good target for nucleophiles which might
be present. Nucleophilic attack results in a charged tetrahedral intermediate. Reformation of the
carbonyl bond ejects a chloride anion.
For example, the reaction of phosgene with an alcohol leads to a charged intermediate
Reforming the carbonyl simultaneously ejects a chloride anion (in keeping with the tetrahedral
valence of carbon). Proton transfer to a fresh molecule of alcohol leads to a neutral intermediate
that still has a very electrophilic carbonyl carbon.
Nucleophilic attack at this position leads to another charged intermediate which can lead back to
a carbonyl compound if the remaining chlorine acts as a leaving group. A final proton transfer to
a fresh alcohol molecule leads to a carbonate.
Because two chlorine atoms and two hydrogen atoms during this mechanism, these by-products
combine to form two equivalents of HCl gas. The major purpose of this type of reaction is to form the carbonate, but HCl gas is produced as a by-product.