Flashcards in Block 4 Deck (48):
Alkenes are saturated/unsaturated
Preparation of alkenes
Usually via elimination reactions
Reagent for dehydration (removal of H2O) alcohol (prep of alkene)
Reagent for dehydrohalogenation (removal of HX) from alkyl halide (prep of alkene)
KOH or NaOH
When is the Saytzeff's rule used
In preparation reactions, when more than one alkene can be formed
The major product is the most substituted alkene, the one with the least number of H's directly attached to the C=C
Reagent for hydrogenation/reduction (alkene reaction)
H2/catalyst (e.g. Pt)
What does hydrogenation occur with?
Syn (same side)/cis
What does halogenation occur with?
Trans (diff sides)/anti because Br/Cl are "big atoms"
Reagent for halogenation (addition of electrophile E+)
When is Markownikoff's rule used?
For addition reactions
What is Markownikoff's rule?
Addition of unsymmetrical reagent to unsymmetrical alkene. The C with the most amount of H's directly attached to it
Preparation of alkynes
Via didehydrohalogenation of dihaloalkanes
Reagent of didehydrohalogenation (for alkynes)
KOH or NaOH in ethanol
What reactions do alkynes undergo
Addition (same as alkenes)
Reagent for hydration (addition of H2O for alkynes)
What reaction do benzene and substitutued benzene undergo?
Usually substitution (because of resonance which prevents them from undergoing addition reaction)
The resonance stabilised carbocation
Steps for preparation of monosubstituted benzenes
1. Generation of electrophiles
2. Electrophillic attack
3. Loss of proton H+
Generation of electrophiles
Usually formed by catalytic action.
Ortho-para strongly activating
-OH, -OR, -NH2, -NR2, NHR
Ortho-para weakly activating
-X (Cl, F, B, I)
Equal amounts of S and R (50:50)
Preparation of alkyl halides (haloalkane)
Addition (Markownikoff's rule applies)
Reagent for 1° and 2° alkyl halides
Reagent for 3° alkyl halides
Halogens are good leaving groups (as they are strong acids and weak bases. E.g.
Poor leaving groups are
H-, NH2, HO-, RO-
What is a carbocation
A molecule that has a C atom bearing 3 bonds (instead of 4) and one positive charge
SN1 unimolecular mechanism is for:
3° alkyl halide (with one species involved)
It is substitution by a nucleophile (which reacts after r.d.s), one species is involved in the r.d.s
SN2 bimolecular mechanism for:
1° and some 2° alkyl halides (with two species involved)
Substitution by a nucleophile, two species involved in the r.d.s
Alkyl halides undergo what type of reaction
Elimination (from an alkyl halide to an alkene)
What reactions do alkenes undergo?
Addition (electron rich)
What is the product for all alkynes except ethyne?
Meta directors strongly deactivating
Meta directors moderately deactivating
CHO2, COR, C---N
Meta directors are ....
very good electron withdrawing groups
How many resonance contributors for ortho-para substitution?
4 resonance contributors
How many resonance contributors for meta substitution?
3 resonance contributors
Which are excellent leaving groups?
I-, Br-, Cl- (weak bases)
Which are poor leaving groups?
H-, NH2-, HO-, RO-
Factors making Sn1 mechanism react faster?
stability (3° is more stable)
leaving group (I- is the best)
E1 unimolecular mechanism is for:
1° and 2°
Proceeds via carbocation intermediate,
What favours elimination over substitution? (3)
Stronger bases, higher temperatures, and non-nucleophilic solvent
E2 bimolecular mechanism is for:
Proton is removed, C=C is formed, C-X is broken all in a single (concerted) step
Vinyl halides can undergo .....
elimination with strong bases to form an alkyne