Nucleophilic substitution

“NS”

Lab #4

Substitution reaction:

A reaction in which one atom, ion, or group is substituted for another.

It is the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair acceptor (the electrophile).

e.g. HO:¯ + CH3CH2-Br → CH3CH2-OH + Br ¯:

Good nucleophile:

1 -have free pair of electrons (-ve charge ).

e.g. ¯OH

2 -the nucleophilicity is decrease while the electronegativity increase.

e.g. ¯ OH > ¯ F , I > Br > Cl > F

3-higher molecular weight will be higher nucleophilicity.

e.g. I > Br > Cl > F

Leaving group:

Any group that can be displaced from a carbon atom.

Good leaving groups are those capable of forming stable ions or molecules upon displacement from the original molecule.

SN2 mechanism (single step)

there is simultaneous formation of the carbon-nucleophile bond and breaking of the carbon-leaving group bond (concerted).

SN2 mechanism (single step)

Energy:

SN2 mechanism (single step)

Energy of the molecules (average energy )

Increase the energy by heating to reach the transition state(kinetic energy).

Eact= kinetic energy + average energy

SN2 mechanism (single step)

Rate:

depend on

1. Temperature

2. Solvent

3. The concentration of the reactants R + Nu

SN2 mechanism (single step)

Example:

CH2CH2CH2CH3

Cl

+ NaI acetone

CH2CH2CH2CH3 + NaCl

I

Example :

SN2 mechanism (single step)

SN1 mechanism(a multi-step)

In an SN1 there is loss of the leaving group

generating an intermediate carbocation which

then undergoes a rapid reaction with the

nucleophile.

This pathway is multi-step process:

step 1: slow loss of the leaving group, LG, to generate a carbocation intermediate (rate determining step).

step 2: rapid attack of a nucleophile on the electrophilic carbocation to form a new bond

SN1 mechanism(a multi-step)

Example:

Step1:

Step2:

SN1 mechanism

Energy:

SN1 mechanism

step1: enough energy must be supplied to break the c-x bond (Eact) & give the carbocation (intermediate).

step2: need lower Eact

SN1 mechanism

Rate:

Depends on

1 -the concentration of the reactant R

2-temperature

3-solvent

SN1 mechanism

Reactivity order of -R:

 

(CH3)3C-  >  )CH3(2CH-   >  CH3CH2-  >  CH3-

3° 2° 1°

Stability: the key step is the loss of the leaving group to form the intermediate carbocation. The more stable the carbocation is, the faster the SN1 reaction will be .

X X

SN1 mechanism

Which substitution mechanism might

operate?

SN1SN2

1-nucleophileNucleophile strength is unimportant .

Strong nucleophile is required

2-substrate3° > 2° not 1°1° > 2° not 3°

3-stepsMulti-stepSingle step

4 -Formation of intermediate carbocation√X

Nucleophilic substitution of alkyl halidesReactions:

SN1:

X= halides (Cl , Br, I )R=

CH3CH2O= -

---> Leaving group

---> electrophile (e acceptor)

---> nucleophile (e donor)

Nucleophilic substitution of alkyl halidesProcedure (SN1 reaction):

1- label 5 clean, dry test tubes from 1to5.2-add 3drops of one of the following halides in test

tube and immediately stopper with each addition:(1) 2-bromobutane.(2) 1-chlorobutane.(3) 2-chloro-2-methylpropane.(4) 2-bromo-2-methylpropane. (5) bromobenzene.

Nucleophilic substitution of alkyl halides

3-in each test tube add 1ml of (1% AgNo3 in ethanol solution) then immediately stopper the test tube.

4-mix thoroughly and record the times for each reaction to form a precipitate or cloudiness.

5-after 5 minutes, place any test tubes that do not contain a precipitate in 100°C water bath, after about 1 minute of heating, cool the test tubes to room temperature and note whether a reaction has occurred .

NOTE: consider the alkyl halide to be unreactive if no turbidity or precipitate appears after 10 min

Nucleophilic substitution of alkyl halides

Test tube no.

Alkyl halide 

Time 

Chemical reaction 

12-bromobutane  

21-chlorobutane  

32-chloro-2-methylpropane  

42-bromo-2-methylpropane  

5Bromobenzene  

THANK YOU