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Chapter 15: Q3P. (page 754)

Question: The central carbon atom of an allene is a member of two double bonds, and it has an interesting orbital arrangement that holds the two ends of the molecule at right angles to each other.

  1. Draw an orbital diagram of allene, showing why the two ends are perpendicular.
  2. Draw the two enantiomers of penta-2,3-diene. A model may be helpful.

Short Answer

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Answer

(a)

(b)

Step by step solution

01

Allene

An allene is a compound containing two double bonds in its structure. The central atom makes a double bond with its two adjacent carbon atoms.The chemical formula for allene is C3H4.

02

Orbital diagram of allene

(a) In the orbital diagram of allene, the central carbon atom makes two pi bonds with the two p orbitals. These two p orbitals are perpendicular to each other, thus forcing the groups present on the ends of an allene compound to become perpendicular. The orbital diagram of an allene is shown as:

Orbital diagram of allene

The orbitals which are shaded are perpendicular to orbitals that are unshaded.

03

Two enantiomers of penta-2,3-diene

The mirror images of penta-2,3-diene that are non-superimposable are known as the enantiomers. The two enantiomers of penta-2,3-diene are shown as:

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Most popular questions from this chapter

Show that the [4+2] Diels-Alder reaction is photochemically forbidden.

Question: When N-bromosuccinimide is added to hex-1-ene in CCl4and a sunlamp is shone on the mixture, three products result.

  1. Give the structures of these three products.
  2. Propose a mechanism that accounts for the formation of these three products.

Propose a mechanism for each reaction, showing explicitly how the observed mixtures of products are formed.

a) 3-methyl-but-2-en-1-ol+HBr→1-bromo-2-ene+3-bromo-3-methyl-1-ene

b)2-methyl-but-3-en-2a-ol+HBr→1-bromo-2-ene+3-bromo-3-methyl-1-enec)cyclopenta-1,3-diene+Br2→3,4-dibromocyclopent-1-ene+3,5-dibromocyclopent-1-ene

d)1-cholorobut-2-ene+AgNO3+H2O→but-2-en-1-ol+but-3-en-2-ol

e)3-cholorobut-1-ene+AgNO3+H2O→but-2-en-1-ol+but-3-en-2-ol

Question: When Br2 is added to buta-1,3- diene at -150C , the product mixture contains 60% of product A and 40% of product B. When the same reaction takes place at 600C , the product ratio is 10% A and 90% B.

a. Propose structures for products A and B (Hint: In many case, an allylic carbocation is more stable than a bromonium ion.)

b. Propose a mechanism to account for formation of both A and B.

c. Show why A predominates at -150C and B predominates at 600C .

d. If you had a solution of pure A, and its temperature were raised to600C , what would you expect to happen ? Propose a mechanism to support your prediction.

(a)Show that the [4+4] cycloaddition of two butadiene molecules to give cycloocta-1,5-diene is thermally forbidden but photochemically allowed.

(b)There is a different, thermally allowed cycloaddition of two butadiene molecules. Show this reaction, and explain why it is thermally allowed. (Hint: Consider the dimerization of cyclopentadiene.)
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