Organic III Flashcards
alkoxide anion
eg. CH3O-
the O has 2 nonbonding e- pairs and a negative charge
forms ether upon substitution
alkynide anion
HC C-
triple bond between two C’s
one nonbonding pair on C with -ve charge
forms alkyne upon substitution
cyanide ion
NC-
nonbonding e- pair on each
triple bond
forms nitrile upon substitution
substitution occurs at a saturated C by a nucleophile IF…
the C bears a good leaving group
nucleophiles that react by substitution have…
an atom with non-bonding e- pairs
for bonding in the product
factors that the strength of a nucleophile depends on
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
common nucleophiles
water alcohols hydroxide ions, alkoxide ions amines halide ions cyanide ions
define leaving group
the group displaced from the organic substrate when it reacts with a nucleophile.
leaving group becomes a nucleophile.
a better leaving group…
is a WEAKER base
H2O > NH3 > OH-
have WEAKER C-X bonds
I- > Br- > Cl- > F-
what organic functional group does NOT react by substitution?
delete this - its not a fact
IDK IF THIS IS A FACT amines
as -NH2 very strong base, not a good leaving group
alkyl halides react by substitution with water to form…
ROH2^+ then ROH after work up
alkyl halides react by substitution with alcohols to form
R2OH+ then ROR (ethers) after work up
alkyl halides react by substitution with amines (1˚ or 2˚) or ammonia to form…
amines RNH2 R2NH R3N (of various ˚)
alkyl halides react by sub with carboxylic salt anions to form
esters
alkyl halides react by sub with cyanide ions to form
nitrile
the C chain becomes larger
rate of substitution haloalkane depends on…
the nature of the haloalkane (1˚, 2˚, 3˚)
the reaction conditions: solvent, temp
define SN2
Substitution, Nucleophilic, bimolecular
rate of an SN2 reaction depends on…
+ equation
the conc of BOTH the substrate AND the nucleophile
because both are involved in the rate-determining step
rate = k [substrate][nucleophile]
what is the rate constant
k
the rate when concentrations = 1 molL-1
Define SN1
Sub, Nucleophilic, unimolecular
rate depends on conc of ONE species
as the nucleophile is NOT involved in the rate-determining step
SN2 mechanism describe
single-step mechanism
= elementary process
intermediate has partial bonds to both the nucleophile and leaving group at the C undergoing substitution
SN2 at a stereogenic centre results in…
inversion
opposite configuration at that centre
For more substituted halides, SN2 is ___ due to greater ____ _____
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
substrates may react by SN2 only if they bear a leaving group attached to a ___ C
saturated
Why are nucleophilic substitution reactions reversible?
Link to yields.
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.
SN1 mechanism
halide reacts in the rate-determining step to produce an intermediate carbocation (slow step) that then reacts with the nucleophile (fast step)
SN1 reaction with substrates with chiral centres products
results in a mixture of stereoisomers.
single stereogenic centre –> racemic mixture (approx 1:1 of both enantiomers)
multiple stereogenic centres –> diastereoisomers
Rate of SN1 is ___ for substrates with more substitution at the C with the leaving group
faster. 3˚ > 2˚ > 1˚
carbocations w/ more bonds to saturated C at C w/ +ve charge are of lower energy, faster ROR
substitution reaction occurring in methanol water mixture
two products formed, as the nucleophiles H2O and CH3OH are of about equal strength.
kinetic order of SN2 and SN1 reactions
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.
how does a polar solvent impact rate of SN1 / SN2?
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
define protic solvent
a type of polar solvent.
protic = H bonded to electronegative atom (eg. methanol, -OH)
used for SN1
what type of solvent is preferred for SN2?
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
what type of solvent is generally used for SN1?>.
methanol or ethanol - polar protic solvents
not water as most org substances are not soluble in water.
make an ester
react an alcohol with a carboxylic acid
“carboxylic acid derivative”
how are carb acids and their derivatives interconverted?
by substitutions
what are the carboxylic acid derivatives? name + general
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
how to make acid anhydride
react an acyl chloride with a carboxylic acid
two carbonyl groups
how to convert esters/amides/acid chlorides to carb acid
hydrolysis
for esters/amides, requires heating
with aq acid or base
alkanoyl chloride + amine –>
amide + ammonium salt
2 mols of the amine are required
a nucleophile that is a stronger base tends towards which type of reaction?
favours ELIMINATION
removal of a proton from C
eg. ethoxide anion (CH3CH2O-)
primary favours sub but 2˚ and 3˚ favour elimination
what are the primary/secondary/tertiary halide reactions in ethanol-water?
uncharged, very weak bases.
primary halides do not react
substitution favoured for secondary and tertiary
E2 reaction
bimolecular elimination
proton removed and leaving group expulsion in single step
E2 reaction stereospecificity
it is stereospecific.
pure diastereoisomer (meso) –> single diastereoisomer of the alkene
E1 path
unimolecular
step 1 = slow ionisation, forming carbocation
step 2 = fast proton transfer from carbocation intermediate to base
E1 and E2 are faster for…
TERTIARY HALIDES > 2˚ > 1˚
as more substituted halides –> more substituted alkenes, greater stability.
