A and D, which are resonance-stabilized, are formed in preference to B and C, which are not. The positive charge of allylic carbocation A is delocalized over two secondary carbons, while the positive charge of carbocation D is delocalized over one secondary and one primary carbon. We therefore predict that carbocation A is the major intermediate formed, and that 4-chloro-2-pentene predominates. Note that this product results from both 1,2 and 1,4 addition.

Allylic halides can undergo slow dissociation to form stabilized carbocations (S_N1 reaction). Both 3-bromo-1-butene and 1-bromo-2-butene form the same allylic carbocation, pictured above, on dissociation. Addition of bromide ion to the allylic carbocation then occurs to form a mixture of bromobutenes. Since the reaction is run under equilibrium conditions, the thermodynamically more stable 1-bromo-2-butene predominates.

1,4 adducts are more stable than 1,2 adducts because disubstituted double bonds are more stable than monosubstituted double bonds (see Chapter 7).

	(a)	(d)
Good dienophiles:
Poor dienophiles:	(b)	(c)
	(e)

Compounds (a) and (d) are good dienophiles because they have electron-withdrawing groups conjugated with a carbon–carbon double bond. Alkene (c) is a poor dienophile because it has no electron-withdrawing functional group. Compounds (b) and (e) are poor dienophiles because their electron-withdrawing groups are not conjugated with the double bond.

Tertiary/primary allylic carbocation A is more stable than secondary/primary allylic carbocation B. Since the products formed from the more stable intermediate predominate, 3,4-dibromo-3-methyl-1-butene is the major product of 1,2 addition of bromine to isoprene. In both cases, the product with the more substituted double bond (1,4 addition product) predominates.

Carbocation D is most stable because it can use the π systems of both the benzene ring and the allylic side chain to delocalize positive charge. 3-Chloro-1-phenyl-1-butene is the major product because it results from cation D and because its double bond can be conjugated with the benzene ring to provide extra stability.

Furthermore, the product of Diels–Alder addition would be impossibly strained, with two sp-hybridized carbons in a six-membered ring.

Two different orientations of the dienophile ester group are possible, and two different products can form.

Most reactive ————————> Least reactive

(NC)₂C=C(CN)₂       >          H₂C=CHCHO          >         H₂C=CHCH₃         >       (CH₃)₂C=C(CH₃)₂

Four electron-                   One electron-                    One electron-               Four electron-

withdrawing groups       withdrawing group           donating group           donating groups

The methyl groups of 2,3-dimethyl-2-butene also decrease reactivity for steric reasons.

Ozone causes oxidative cleavage of the double bonds in rubber and breaks the polymer chain.

Each alkyl substituent causes an increase in λ_max of 3–6 nm.

In Problem 14.41, we concluded that one alkyl group increases λ_max of a conjugated diene by approximately 5 nm. Since 2,3-dimethyl-1,3,5-hexatriene has two methyl substituents, its UV λ_max should be about 10 nm longer than the λmax of 1,3,5-hexatriene.

(a) The three pi-bonds that are conjugated each have two p-atomic orbitals. Thus, there are six p-atomic orbitals in the conjugated system.

(b) The total number of molecular orbitals must equal the number of atomic orbitals used to create them. Therefore there are six molecular orbitals. (Ψ₁, Ψ₂, Ψ₃, Ψ₄, Ψ₅, Ψ₆)

(c) The bonding molecular orbitals are lower in energy than the p-atomic orbital energies. Thus, there are three bonding orbitals (Ψ₁, Ψ₂, Ψ₃).

(d) The antibonding molecular orbitals are higher in energy than the p-atomic orbital energies. Thus, there are three antibonding orbitals (Ψ₄*, Ψ₅*, Ψ₆*; antibonding orbitals are denoted with an *).

(e) Because orbitals fill from lowest energy to highest energy and there are six electrons in the bonding Ψ₁, Ψ₂ and Ψ₃ molecular orbitals.

(f) The electron would move from the Ψ₃ (the highest occupied molecular orbital; HOMO) to Ψ₄* (the lowest unoccupied molecular orbital; LUMO).

2,4-Hexadiene can easily be distinguished from the other two isomers because it is the only isomer that absorbs in the UV region. The other two isomers show significant differences in their ¹H and ¹³C NMR spectra and can be identified by either technique.

Conjugation with the                        Reaction with HCl yields                    Addition of Cl– leads to the oxygen lone pair electrons              a cation intermediate that                  observed product. makes the double bond                   can be stabilized by the oxygen more nucleophilic                              electrons.

There are two reasons why the other regioisomer is not formed: (1) Carbon 1 is less nucleophilic than carbon 2; (2) The cation intermediate that would result from protonation at carbon 1 can’t be stabilized by the oxygen electrons and does not form.

The ozonolysis fragments from myrcene are 2-oxopentanedial (five carbon atoms), acetone (three carbon atoms), and two equivalents of formaldehyde (one carbon atom each). Putting these fragments together in a manner consistent with the data gives the following structural formula for myrcene:

The stereochemistry of the product resulting from Diels–Alder reaction of the (2E,4Z) diene differs at the starred carbon from that of the (2E,4E) diene. Not only is the stereochemistry of the dienophile maintained during the Diels–Alder reaction, the stereochemistry of the diene is also maintained.