Chapter 7

Question 1

Unsaturation

Answer 1

A structural element that decreases the number of hydrogens in the molecule by two

•Multiple bonds and rings are elements of unsaturation

Question 2

How do you calculate degree of saturation?

Answer 2

To calculate: Find number of hydrogens if they were saturated (2n +2; n is # of carbons), subtract the actual number of hydrogens, then divide by 2

Question 3

Halogens in unsaturation

Answer 3

Halogens replace hydrogen atoms in hydrocarbons, so when calculating unsaturations, count halides as hydrogen atoms

Question 4

Oxygen in unsaturation

Answer 4

Oxygen does not change the C:H ratio, so ignore oxygen in the formula

Question 5

Nitrogen in unsaturation

Answer 5

Nitrogen is trivalent, so it acts like half a carbon. Add the number of nitrogen atoms when calculating unsaturations

Question 6

IUPAC

Answer 6

• Find the longest continuous carbon chain that includes the double-bonded carbons
• -ane changes to -ene
• Number the chain so that the double bond has the lowest possible number.
• In a ring, the double bond is assumed to be between Carbon 1 and Carbon 2

Question 7

Z Configuration

Answer 7

•If high priority groups are on the same side, the name is Z (for zusammen)

Question 8

E configuration

Answer 8

• If high priority groups are on opposite sides, the name is E (for entgegen)

Question 9

Relative Stabilities of Alkenes

Answer 9

(most stable) tetrasubstituted > trisubstituted > disubstituted > monosubstituted

Question 10

Disubstituted Isomers

Answer 10

cis < geminal < trans isomer (most stable)

Question 11

Bredt’s Rule

Answer 11

•A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms

Question 12

Dehydrohalogenation by the E2 Mechanism

Answer 12

• Strong base abstracts H+ as double bond forms and X- leaves from the adjacent carbon
• Tertiary and hindered secondary alkyl halides give good yields

Question 13

Zaitsev Product

Answer 13

more substituted alkene

Question 14

Hofmann Product

Answer 14

less substituted alkene

Question 15

How do bulky bases affect elimination reactions?

Answer 15

Bulky bases produce more of the Hofmann product

If the substrate is prone to substitution, a bulky base can minimize the amount of substitution.

Large alkyl groups on a bulky base hinder its approach to attack a carbon atom (substitution), yet it can easily abstract a proton (elimination).

Question 16

Why use a bulky base?

Answer 16

Where both substitution and elimination are possible, using a bulky base gives a clean elimination result.

Question 17

Dehalogenation of Vicinal Dibromides

Answer 17

Similar to E2 reaction, except Br2 is removed from adjacent carbons instead of H–Br

Bromines must also be anti-coplanar (E2)

Use NaI in acetone, or Zn in acetic acid

Question 18

What type of reactant/solvent must be used for dehalogenation of vicinal bromides?

Answer 18

Use NaI in acetone,

or Zn in acetic acid

Question 19

For SN1, what conditions favor elimination over substitution?

Answer 19

High temperature and weak/bulky bases will favor elimination over substitution.

Question 20

What type of mechanism is dehydration of alchohols?

Answer 20

E1

Question 21

Dehydration of Alchohols Summary

Answer 21

• Use concentrated sulfuric or phosphoric acid to protonate the alcohol, and make it a good leaving group
• Rest of the reaction is the same as an E1 reaction
• To increase the yield of the reaction, you can remove the low-boiling alkene as it forms to shift the equilibrium (the reaction is reversible)

Mildly acidic solvent: favors elimination

Question 22

Critical (acid/base) rules for mechanisms

Answer 22

Critical Rules for Mechanisms:

• Do not form basic species (neg. charge) in acidic media
• Do not form acidic species in basic media

Question 23

Dehydration of Alchohols Side Reactions

Answer 23

–Substitution products are formed (SN1-nucleophile adds to carbocation)

–Amount of substitution depends on temperature and nature of solvent

–Low temperature: substitution favored

–High Temperature: elimination favored

Question 24

How does s character affect electronegativity?

Answer 24

Greater s character leads to an increase in electronegativity

Question 25

How does heat of hydrogenation relate to alkene stability

Answer 25

• The more substituted the double bond, the lower its heat of hydrogenation = the more stable

Question 26

How is heat of hydrogenation determined?

Answer 26

It is the heat given off when an alkene becomes an alkane (gain of 2 hydrogens)

Question 27

E1 Dehydrohalogenation

Answer 27

Occurs in polar protic solvents or under acidic conditions. There are no stereochemical requirements since there are multiple steps in the reaction.

Prone to rearrangement

Only forms Zaitsev product

Accompanied by SN1

Question 28

What type of acid is used for dehydration of alchohols?

Answer 28

Typically conducted in sulfuric acid (H2SO4) and/or phosphoric acid (H3PO4).