Organic III Flashcards by Joanna Li

alkoxide anion

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

forms ether upon substitution

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alkynide anion

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

forms alkyne upon substitution

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cyanide ion

NC-
nonbonding e- pair on each
triple bond

forms nitrile upon substitution

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substitution occurs at a saturated C by a nucleophile IF…

the C bears a good leaving group

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nucleophiles that react by substitution have…

an atom with non-bonding e- pairs

for bonding in the product

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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

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common nucleophiles

water
alcohols 
hydroxide ions, alkoxide ions 
amines
halide ions 
cyanide ions

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define leaving group

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

leaving group becomes a nucleophile.

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a better leaving group…

is a WEAKER base
H2O > NH3 > OH-

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

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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

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alkyl halides react by substitution with water to form…

ROH2^+ then ROH after work up

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alkyl halides react by substitution with alcohols to form

R2OH+ then ROR (ethers) after work up

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

amines 
RNH2 
R2NH
R3N 
(of various ˚)

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alkyl halides react by sub with carboxylic salt anions to form

esters

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alkyl halides react by sub with cyanide ions to form

nitrile

the C chain becomes larger

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rate of substitution haloalkane depends on…

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

the reaction conditions: solvent, temp

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define SN2

Substitution, Nucleophilic, bimolecular

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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]

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what is the rate constant

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.