Ch 7: Alkyl Halides and Nucleophilic Substitution

Question 1

What is the most important factor in predicting what reaction will occur with an alkyl halide?

Answer 1

Whether it is primary, secondary, or tertiary

[7.1]

Question 2

What is a vinyl halide?

Answer 2

A halide in which the halogen is bonded to a carbon-carbon double bond (and thus, the C bonded to the halogen is sp² hybridized).

Question 3

What is an aryl halide?

Answer 3

A halide in which the halogen atom is bonded to a benzene ring; (the C is sp² hybridized).

[7.1]

Question 4

Note: Vinyl halides and aryl halides do not undergo the reactions presented in Ch 7 (nucleophilic substitution).

Question 5

Allylic halides have their halogen atoms bonded to the carbon atom ________ to a C=C double bond.

Answer 5

adjacent

Question 6

Benzylic halides have their halogen atom bonded to the carbon atom ________ to the benzene ring.

Answer 6

Question 7

Since allylic and benzylic halides have their halogen atoms bonded to ___ hybridized carbon atoms, they DO undergo the reactions discussed in chapter 7.

Answer 7

sp³

[7.1]

Question 8

Why can’t alkyl halides hydrogen bond even if the halogen atom is fluorine?

Answer 8

Because they have only C-C, C-F, or C-H bonds, and you would need a H-F atom in order to hydrogen bond!

[7.3]

Question 9

Alkyl halides are ______ polar bonds, and do exhibit dipole-dipole forces.

Answer 9

weakly

[7.3]

Question 10

What are alkyl halides soluble in?

Answer 10

Despite being weakly polar, they are insoluble in water, and soluble in organic solvents.

Question 11

The electronegative halogen atom X of an alkyl halide creates a polar C-X bond, making the carbon atom _________ ________. The chemistry of alkyl halides is determine by this bond.

Answer 11

electron deficient

[7.5]

Question 12

What type of reactions can alkyl halides undergo with nucleophiles?

Answer 12

substitution reactions

[7.5]

Question 13

What type of reaction can alkyl halides undergo with Bronsted-Lowry bases?

Answer 13

elimination reactions

Question 14

In comparing two leaving groups, the better leaving group is the ______ ____.

Answer 14

weaker base

[7.7]

Question 15

Left-to-right across a row of the periodic table, basicity __________, so leaving group ability ________.

Down a column of the periodic table, basicity _________, so leaving group ability _________.

Answer 15

decreases; increases

decreases; increases

[7.7]

Question 16

All good leaving groups are weak bases with strong _________ _____ having low pK_a values.

Answer 16

conjugate acids

[7.7]

Question 17

Why does nucleophilic substitution NOT occur with any of the leaving groups in table 7.3?

Answer 17

Because these leaving groups are STRONG bases.

Question 18

Equilibrium favors the products of nucleophilic substitution when the leaving group is a ______ ____ than the nucleophile.

Answer 18

weaker base

Question 19

In predicting the reactivity of alkyl halides in SN2 reactions, what must we consider?

For example, which is most/least reactive when comparing similarly sized primary/secondary/tertiary alkyl halides?

Answer 19

We must consider steric hindrance.

A primary halide will be the most reactive, while a tertiary halide will be the least reactive.

Question 20

Nucleophiles and bases are structurally similar; both have a lone pair or a pi bond. They differ in….

Answer 20

….what they attack.

p. 264

Question 21

Note: We use the word ‘base’ to mean Bronsted-Lowry base and the word nucleophile to mean a Lewis base that reacts with electrophiles OTHER than protons.

Answer 21

p. 264

Question 22

Bases attack _______. Nucleophiles attack other ________-deficient atoms (usually carbons).

Answer 22

protons

electron

p. 264

Question 23

Although it is generally true that a strong base is a strong nucleophile, nucleophile size and steric factors can sometimes change this relationship. Nucleophilicity parallels basicity in 3 instances:

Answer 23

1. for two nucleophiles with the same nucleophilic atom, the stronger base is the stronger nucleophile.
2. a negatively charged nucleophile is always stronger than its conjugate acid.
3. Right-to-left across a row of the periodic table, nucleophilicity increases as basicity increases.

Question 24

Nucleophilicity does NOT parallel basicity when steric hindrance becomes important. This decrease in reactivity results from the presence of bulky groups at the site of a reaction. All steric effects arise bc two atoms cannot….

Answer 24

….occupy the same space.

p. 264

Question 25

Why does steric hindrance decrease nucleophilicity, but not basicity?

Answer 25

Bc bases pull off small, easily accessible protons.

p. 264

Question 26

What are sterically hindered bases that are poor nucleophiles called?

Answer 26

nonnucleophilic bases

p. 265

Question 27

Polar protic solvents are capable of intermolecular hydrogen bonding. Because of this they solvate both cations and anions well. Cations are solvated by __________ interactions; anions are solvated by _________.

Answer 27

ion-dipole

hydrogen bonding

p. 265