Ch 7: Alkyl Halides and Nucleophilic Substitution Flashcards
What is the most important factor in predicting what reaction will occur with an alkyl halide?
Whether it is primary, secondary, or tertiary
[7.1]
What is a vinyl halide?
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).
What is an aryl halide?
A halide in which the halogen atom is bonded to a benzene ring; (the C is sp2 hybridized).
[7.1]
Note: Vinyl halides and aryl halides do not undergo the reactions presented in Ch 7 (nucleophilic substitution).
Allylic halides have their halogen atoms bonded to the carbon atom ________ to a C=C double bond.
adjacent
Benzylic halides have their halogen atom bonded to the carbon atom ________ to the benzene ring.
Since allylic and benzylic halides have their halogen atoms bonded to ___ hybridized carbon atoms, they DO undergo the reactions discussed in chapter 7.
sp3
[7.1]
Why can’t alkyl halides hydrogen bond even if the halogen atom is fluorine?
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]
Alkyl halides are ______ polar bonds, and do exhibit dipole-dipole forces.
weakly
[7.3]
What are alkyl halides soluble in?
Despite being weakly polar, they are insoluble in water, and soluble in organic solvents.
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.
electron deficient
[7.5]
What type of reactions can alkyl halides undergo with nucleophiles?
substitution reactions
[7.5]
What type of reaction can alkyl halides undergo with Bronsted-Lowry bases?
elimination reactions
In comparing two leaving groups, the better leaving group is the ______ ____.
weaker base
[7.7]
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 _________.
decreases; increases
decreases; increases
[7.7]
All good leaving groups are weak bases with strong _________ _____ having low pKa values.
conjugate acids
[7.7]
Why does nucleophilic substitution NOT occur with any of the leaving groups in table 7.3?
Because these leaving groups are STRONG bases.
Equilibrium favors the products of nucleophilic substitution when the leaving group is a ______ ____ than the nucleophile.
weaker base
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?
We must consider steric hindrance.
A primary halide will be the most reactive, while a tertiary halide will be the least reactive.
Nucleophiles and bases are structurally similar; both have a lone pair or a pi bond. They differ in….
….what they attack.
p. 264
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.
p. 264
Bases attack _______. Nucleophiles attack other ________-deficient atoms (usually carbons).
protons
electron
p. 264
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:
- for two nucleophiles with the same nucleophilic atom, the stronger base is the stronger nucleophile.
- a negatively charged nucleophile is always stronger than its conjugate acid.
- Right-to-left across a row of the periodic table, nucleophilicity increases as basicity increases.
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….
….occupy the same space.
p. 264
