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Chem Oly Pre-Camp > Organic III > Flashcards

Organic III Flashcards

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1
Q

alkoxide anion

A

eg. CH3O-
the O has 2 nonbonding e- pairs and a negative charge

forms ether upon substitution

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2
Q

alkynide anion

A

HC C-
triple bond between two C’s
one nonbonding pair on C with -ve charge

forms alkyne upon substitution

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3
Q

cyanide ion

A

NC-
nonbonding e- pair on each
triple bond

forms nitrile upon substitution

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4
Q

substitution occurs at a saturated C by a nucleophile IF…

A

the C bears a good leaving group

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5
Q

nucleophiles that react by substitution have…

A

an atom with non-bonding e- pairs

for bonding in the product

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6
Q

factors that the strength of a nucleophile depends on

A

charge
same atom, anion>molecule
eg. HO- > H2O

size
larger - better
I- > Br- > Cl- > F-

electronegativity
less electronegative atoms hold non-bonding e- less strongly, better nucleophiles
eg. NH3 > H2O

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7
Q

common nucleophiles

A 
water
alcohols 
hydroxide ions, alkoxide ions 
amines
halide ions 
cyanide ions
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8
Q

define leaving group

A

the group displaced from the organic substrate when it reacts with a nucleophile.

leaving group becomes a nucleophile.

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9
Q

a better leaving group…

A

is a WEAKER base
H2O > NH3 > OH-

have WEAKER C-X bonds
I- > Br- > Cl- > F-

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10
Q

what organic functional group does NOT react by substitution?

delete this - its not a fact

A

IDK IF THIS IS A FACT amines

as -NH2 very strong base, not a good leaving group

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11
Q

alkyl halides react by substitution with water to form…

A

ROH2^+ then ROH after work up

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12
Q

alkyl halides react by substitution with alcohols to form

A

R2OH+ then ROR (ethers) after work up

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13
Q

alkyl halides react by substitution with amines (1˚ or 2˚) or ammonia to form…

A 
amines 
RNH2 
R2NH
R3N 
(of various ˚)
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14
Q

alkyl halides react by sub with carboxylic salt anions to form

A

esters

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15
Q

alkyl halides react by sub with cyanide ions to form

A

nitrile

the C chain becomes larger

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16
Q

rate of substitution haloalkane depends on…

A

the nature of the haloalkane (1˚, 2˚, 3˚)

the reaction conditions: solvent, temp

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17
Q

define SN2

A

Substitution, Nucleophilic, bimolecular

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18
Q

rate of an SN2 reaction depends on…

+ equation

A

the conc of BOTH the substrate AND the nucleophile
because both are involved in the rate-determining step

rate = k [substrate][nucleophile]

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19
Q

what is the rate constant

A

k

the rate when concentrations = 1 molL-1

20
Q

Define SN1

A

Sub, Nucleophilic, unimolecular

rate depends on conc of ONE species
as the nucleophile is NOT involved in the rate-determining step

21
Q

SN2 mechanism describe

A

single-step mechanism
= elementary process

intermediate has partial bonds to both the nucleophile and leaving group at the C undergoing substitution

22
Q

SN2 at a stereogenic centre results in…

A

inversion

opposite configuration at that centre

23
Q

For more substituted halides, SN2 is ___ due to greater ____ _____

A

SLOWER

Steric crowding

the presence of larger groups increases crowding in the transition state and raises the energy. Higher activation energy –> slower ROR

This makes tertiary halides largely unreactive toward substitution

24
Q

substrates may react by SN2 only if they bear a leaving group attached to a ___ C

A

saturated

25
Q

Why are nucleophilic substitution reactions reversible?

Link to yields.

A

the product arising from the leaving group has a non-bonding pair, capable of acting as a nucleophile

good yield if reactant nucleophile is stronger than the product nucleophile.

26
Q

SN1 mechanism

A

halide reacts in the rate-determining step to produce an intermediate carbocation (slow step) that then reacts with the nucleophile (fast step)

27
Q

SN1 reaction with substrates with chiral centres products

A

results in a mixture of stereoisomers.

single stereogenic centre –> racemic mixture (approx 1:1 of both enantiomers)

multiple stereogenic centres –> diastereoisomers

28
Q

Rate of SN1 is ___ for substrates with more substitution at the C with the leaving group

A

faster. 3˚ > 2˚ > 1˚

carbocations w/ more bonds to saturated C at C w/ +ve charge are of lower energy, faster ROR

29
Q

substitution reaction occurring in methanol water mixture

A

two products formed, as the nucleophiles H2O and CH3OH are of about equal strength.

30
Q

kinetic order of SN2 and SN1 reactions

A

SN2: for k[nucleophile][substrate]
first order with respect to each reactant, as the exponent is 1.
sum = second order reaction overall

SN1 is first order.

31
Q

how does a polar solvent impact rate of SN1 / SN2?

A

if the product(s) have higher charge, ROR higher in more polar solvent (as energy lowered by solvation).

SN1: RDS is heterolytic cleavage –> charged particles.
increase solvent polarity = increase ROR

SN2: products of RDS are also an anion + an uncharged product
polarity has NO EFFECT on rate

32
Q

define protic solvent

A

a type of polar solvent.
protic = H bonded to electronegative atom (eg. methanol, -OH)

used for SN1

33
Q

what type of solvent is preferred for SN2?

A

polar aprotic solvents
solvate cations in preference to anions, leaving nucleophilic atom more exposed and therefore more reactive.

eg. dimethylformamide (DMF) 
dimethyl sulfoxide (DMSO) 
propanone
34
Q

what type of solvent is generally used for SN1?>.

A

methanol or ethanol - polar protic solvents

not water as most org substances are not soluble in water.

35
Q

make an ester

A

react an alcohol with a carboxylic acid

“carboxylic acid derivative”

36
Q

how are carb acids and their derivatives interconverted?

A

by substitutions

37
Q

what are the carboxylic acid derivatives? name + general

A

RCOX
X has electroneg atom bonded to carbonyl C=O

Acyl chloride 
anhydride 
ester 
amine 
carboxylic acid

in order of most to least reactive to nucleo sub

38
Q

how to make acid anhydride

A

react an acyl chloride with a carboxylic acid

two carbonyl groups

39
Q

how to convert esters/amides/acid chlorides to carb acid

A

hydrolysis
for esters/amides, requires heating
with aq acid or base

40
Q

alkanoyl chloride + amine –>

A

amide + ammonium salt

2 mols of the amine are required

41
Q

a nucleophile that is a stronger base tends towards which type of reaction?

A

favours ELIMINATION
removal of a proton from C

eg. ethoxide anion (CH3CH2O-)
primary favours sub but 2˚ and 3˚ favour elimination

42
Q

what are the primary/secondary/tertiary halide reactions in ethanol-water?

A

uncharged, very weak bases.

primary halides do not react
substitution favoured for secondary and tertiary

43
Q

E2 reaction

A

bimolecular elimination

proton removed and leaving group expulsion in single step

44
Q

E2 reaction stereospecificity

A

it is stereospecific.

pure diastereoisomer (meso) –> single diastereoisomer of the alkene

45
Q

E1 path

A

unimolecular

step 1 = slow ionisation, forming carbocation
step 2 = fast proton transfer from carbocation intermediate to base

46
Q

E1 and E2 are faster for…

A

TERTIARY HALIDES > 2˚ > 1˚

as more substituted halides –> more substituted alkenes, greater stability.

Chem Oly Pre-Camp (14 decks)

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