Nucleophilic Substitution Swapping - PowerPoint Presentation

Slide1

Nucleophilic Substitution Swapping

Generic Equation: SwapR-X + Nu:  R-Nu + :X-The problem lies in the mechanism.Slide2

Importance of Alkyl Halides

Precursor to many othersSlide3

NUCLEOPHILIC SUBSTITUTION

Theory

- nucleophile means ‘liking positive’

- a

dipole is induced (by a Nu:)

in the C-X bond and it is polar

polar bond, why is it polar with chlorine?

Answer: ∆E=0.4, electronegativity difference

OH¯ CN¯ NH

3

H

2

O

These are the 4 Nu: NaOH, NaCN the 2 moleculars are as isSlide4

SN

2

:NUCLEOPHILIC SUBSTITUTION

MECHANISM

MECHANISM: 2 Steps

The NU: electrons attacks the slightly positive carbon atom

The polar bond breaks unevenly (

heterolytic) and Br- released

Back

attack

Polar Bond

Note/Points

:

-attack from back (electronegative halogen will forces this)

-show the polar bond

Both e-

To halogen

2 productsSlide5

SN

2All 10 (primary)halides react as SN2

reactions.

The 2 does NOT mean steps

S = substitution

N = Nucleophile

2 = second order (both reactants determine rate)

Note: this is for primary halides, tertiary will under go SN1 as we will see later, secondary will do both – but we are not responsible to know about secondary reactionsSlide6

1O

Haloalkanes SN2https://www.youtube.com/watch?v=h5xvaP6bIZISlide7

There are 4 Substitutions

1) WITH HYDROXIDE C2

H

5

Br(l) + OH-

(aq)

-->C2H5OH(l) +Br-(aq)2) WITH A CYANIDE C2

H5Br(l) + CN- (aq/alc) —>C

2H5CN + Br-(aq) 3) WITH 2 AMMONIAS C2H5Br(l) + 2NH3(aq/alc)—> C

2H5NH2 + NH4Br4) WITH H2O (solvolysis) solvent and reactant at same time C2H5Br(l) + H2O

(l) —> C2H5OH(l) + HBr (aq)Slide8

Chapter 6

8

S

N

2 Energy Diagram

The S

N

2 reaction is a one-step reaction.

Transition state is highest in energy. Slide9

Substitution Reactions

: The Nu: will be one of these 4 reactants

These four reactants will be

similar in result and mechanism

They will ALL be refluxed

Note: NaOH or KOH, NaCN etc is the source of

The

OH¯ , and CN¯ above as they are ionic

But we can ignore the metals as spectators

NaOH ---> Na⁺ + OH⁻

OH¯ CN¯ NH3 H2OSlide10

AQUEOUS

HYDROXIDE



ALCOHOLS

Reagent

Aqueous*

NaOH

or KOH

(aq) (OH¯)Conditions Reflux in aqueous solution (SOLVENT IS IMPORTANT)Product AlcoholNucleophile hydroxide ion ( : OH¯)Equation e.g. C2H5

Br(l) + Na OH-(

aq) ——> C2H

5OH(l) + Na Br-(

aq

)

Spectator Spectator

Mechanism

STEP 1 Nu: attack

STEP 2

:

Br- forms and leaves

*

WARNING

It is important to quote the solvent when answering questions. Elimination takes place when ethanol is the solvent - SEE LATER

NUCLEOPHILIC SUBSTITUTIONSlide11

CYANIDE makes Nitriles AND makes chain LONGER

Reagent

Alcohol/ Aqueous,

NaCN

/ KCN

Conditions

Reflux in aqueous , alcoholic solution

Product

Nitrile (cyanide)

Nucleophile cyanide ion (CN¯)

Equation e.g. C2

H5Br + Na CN (

aq/alc) ——> C2

H

5

CN +

Na

CN-(

aq

)

Mechanism

1-bromoethane propanenitrile Br-

Importance extends the carbon chain by one carbon atom, called propanenitrile .

NUCLEOPHILIC SUBSTITUTIONSlide12

NUCLEOPHILIC SUBSTITUTION

AMMONIA makes an amine

Reagent

Aqueous, alcoholic ammonia (

in EXCESS, or 2 Ammonia’s

)

Conditions

Reflux in aqueous , alcoholic solution under pressure

Product

AmineNucleophile Ammonia (NH3 (aq/alc) )Equation

e.g. C2H5

Br + 2NH3 (aq /

alc) ——> C2H5

NH

2

+ NH

4

Br

(i)

C

2

H5Br + NH3 (aq / alc) ——> C

2H5NH3 + HBr (ii) C2H5NH3

+ NH3 (aq / alc) ——> C2H5NH2 + NH4Br

_______________________________________________

C

2

H

5

Br + 2NH

3

(

aq

/

alc

)

——> C

2

H

5

NH

2

(l)

+ NH

4

Br

(

aq

/

alc

)

Mechanism

Ethaneamine

Bromide

Slide13

NUCLEOPHILIC SUBSTITUTION

AMMONIA

Why excess ammonia?

The second ammonia molecule ensures the removal of the H to make an amine. A large excess ammonia ensures that further substitution doesn’t take place.

Problem

Amines are also nucleophiles (lone pair on N) and can attack another molecule of haloalkane to produce a 2° amine. This too is a nucleophile and can react further producing a 3° amine and, eventually an ionic quarternary ammonium salt.

..

C

2

H

5

NH2 + C2H

5Br ——> HBr + (C2H5

)

2

NH

diethylamine, a 2° amine

..

(C

2

H

5)2NH + C2H5Br ——> HBr + (C2H5)

3N triethylamine, a 3° amine(C2H5)3N + C2H5Br ——> (C2H5)

4N+ Br¯ tetraethylammonium bromide a quaternary (4°) saltSlide14

NUCLEOPHILIC SUBSTITUTION

WATER Makes Alcohols

Details

A similar reaction to that with OH¯ takes place with water.

It is

slower as water is a poor nucleophile

.

Water may be looked at as HOH THE OH AND BR SWAP

Equation

C

2H5Br(l) + H2

O(l) ——> C2H5OH(l) + HBr(aq)

We show the product as HBr and not just Br- because it is molecular and the H in the water is not an ion like Na+ or K+. As well, H3O+ forms, which we rewrite as H+ and the water from the H3O+ is part of the (aq)Slide15

SN1

V SN2There is very little difference in these reactions.Both have 2 steps and the same product , the same reactants. Hard to see any difference S = substitution (both will swap)N= Nucleophile (both will use the same Nu:

1 =Rate of reaction (depends on 1 reactant)

called 1st order

2 =

Rate of reaction (depends on 2 REACTANTS) called 2nd order called 1st order Slide16

Steric

Hinderance With a tertiary halogenoalkane, NU: attack is impossible. The back of the molecule is completely cluttered with CH3 groups.It has

to go by an alternative mechanism

.Slide17

Tertiary Haloalkenes

SN1Steric hindrance means the Nu: cannot approach and has no effect on the reactions first step

The

[Nu

:] nucleophile has

no effect.

Only the [haloalkane] affect the reaction rate. Thus it is first order. rate = k [haloalkane] 1Slide18

Mechanism for3o

Halides SN1

Carbocation Intermediate

SLOW (RDS)

STEP

FAST

STEPhttp://ibchem.com/IB16/10.34.htmSlide19

Chapter 6

19

S

N

1 Energy Diagram

Forming the carbocation is an endothermic step.

Step 2 is fast with a low activation energy.Slide20

Film Clip 30

HaloalkanesSN2 (single step)https://www.youtube.com/watch?v=JmcVgE2WKBESlide21

Secondary Halides

They undergo both SN1 and SN2 reactions so on an exam if you are given a secondary halide, either mechanism will receive full credit in marksSlide22

LAST REACTION

ELIMINATIONSlide23

3 WAYS to Favor Elimination

To favor elimination rather than substitution use:1) More heat2) More concentrated hydroxide in ALCOHOL3) pure ethanol as the solventSlide24

24

The

:

OH- is a BASE, in alcohol and takes a hydrogen (H+) to form water. This sets off a cascade where a C=C forms and the halide ion is lost. (as always Reflux)

EliminationSlide25

Elimination Via OH-

BASEStep 1: The :OH- is NOT a nucleophile, it is a BASE, and attacks HStep 2: The H+ leave and e- pair make C=CStep 3: The halogen (here as :Br-) leaves GENERIC CASCADING REACTION

+ H

2

O + :Br-

+ H

2O + :Br-

An ExampleSlide26

ELIMINATION

Reagent

Alcoholic

sodium

(or potassium)

hydroxide

Conditions

Reflux

in alcoholic solution, ALL alcoholProduct Alkene

Mechanism EliminationEquation C

3H7Br + NaOH(

alc) ——> C3H

6

+ H

2

O +

NaBr

Mechanism

the

OH¯ ion acts as a base

and picks up a proton

the proton comes from a carbon atom next to that bonded to the halogen the electron pair left moves to form a second bond between the carbon atoms

the halogen is displaced overall there is ELIMINATION of HBr.Complication:Cis / Trans with unsymmetrical halalkanes, you get mixture of productsSlide27

ELIMINATION

Complication

If

the haloalkane is unsymmetrical

a

mixture of isomeric alkene products is obtained.

but-1-ene (Minor Product)

but-2-ene (Major Product)

can

exist as cis and trans isomers

2-bromobutane

2-bromobutaneSlide28

ELIMINATION v. SUBSTITUTION

The products of reactions between haloalkanes and OH¯ are influenced by the solvent

SOLVENT

ROLE OF OH

–

MECHANISM

PRODUCT

WATER

NUCLEOPHILE

SUBSTITUTION

ALCOHOL

ALCOHOL

BASE

ELIMINATION

ALKENE

Modes of attack

Aqueous

soln

OH

¯ acts as a nucleophile

Alcoholic

soln

OH

¯ acts as a base

(A BASE IS A PROTON ACCEPTOR)

Both reactions take place at the same time but by varying

the solvent you can influence which mechanism dominates. Slide29

Elimination Clips

https://www.learnnext.com/CBSE/Class-12/Chemistry/Haloalkanes-and-Haloarenes/Chemical-Reactions-Of-Alkyl-Halides-Elimination-Reactions/L-1804203.htmFranklychemhttps://www.youtube.com/watch?v=o3lk77sdFHYSlide30

Aprotic solvents

ε

Protic solvents

ε

Hexane

1.9

Acetic acid

6.2

Benzene

2.3

1-Methyl-2-propanol

11

Diethyl ether

4.3

Ethanol

34.3

Chloroform

4.8

Methanol

33.6

Hexamethylphosphoramide (HMPT)

30

Formic acid

58.0

Dimethyl formamide (DMF)

38

Water

80.4

Dimethyl sulfoxide (DMSO)

48

The ability of solvents to stabilize ions through solvation is directly associated

with their polarity. Polar solvents, such as water, can stabilize the ions 3X more

than alcohol

(see chart)

Methanol can stabilize ions through solvation, but water is best at controlling

ions. The :OH- is a super strong ion and needs controlling. Alcohol controls less

and is better to dissolve weak :Nu’s such as NH

3

and CN, as they do not need

to be controlled like OH- as they are weak ions