CPR Organic More Nucleophilic Substitution

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Chemical Processes and Reactions - Organic Chemistry - More Nucleophilic Substitution

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Substitution at Tertiary Centres - Unimolecular Me

A different mechanism must be considered for tertiary alkyl halides due to steric hindrance.

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Rate = k.[Me CCl]         (rate is only proportional to the conc. of Me CCl)

The mechanism is consistent with the rate determining step.

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Energy Profile of Substitution at Tertiary Centres

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Energy of intermediate cation and structure are both very similar to ts1.

The activation energy is lower for more stable intermediates.

The more stable the cation intermediate the faster the rate in the mechanism.

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Intermediate Structure

Intermediate carbocation is planar and sp2 hybridised.

The carbocation can be attacked from either side as it gives the same product.

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Carbo cations can be stabilised by:

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.                     Inductive Donation                              Hyperconjugation

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Mechanism Nomenclature

The rate determining step is unimolecular and is therefore called Substitution Nucleophilic Unimolecular-1. This denotes S 1.

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S 1 Features:

Two step reaction; the leaving group leaves first in a slow rate determining step. The intermediate is depicted. Racemisation of stereogenic centres.

Rate = k.[Substrate]

Only alkyl halides which can give good carbocation react by this mechanism

no S 1 at 1° centre since C+ stability is Tertiary> Secondary>> Primary 

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Rates of S 1 Substitutions and Cation Stability

Consider the reaction:

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    RBr                    Relative Rate

MeBr                               1

EtBr                                2

PrBr                               43

tert- butylBr                    10

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More Stabilising Carbocations

Resonance stabilises carbocations in this way:

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The C+ is delocalised over 4 carbons at the cost of aromaticity.

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An adjacent lone pair stabilises the carbocation in this way:

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S 1 Reactivity

1° <<  2° ≈ CH =CH-CH (+) < PhCH (+) ≈ 3°

There is no reaction via the S 1 system at (unstable) primary centres 

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Benzylic Substrates Activated for S 1 and S 2

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In benzene the S 1 reaction is triggered by the delocalisation of the cation.

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S 2 is activated by the π system

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Electronic Effects

S 2 that is 100 X faster than ethyl bromide. An orbital overlap with the π C=C, this stabilises the transition state.

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S 2 that is 1000000 X faster than a primary alkyl halide. An orbital overlap with the π* C=O, this stabilises the transition state.

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Rationale

When we mix orbitals we get two new ones.

This works for bonding but can also work to stabilise transition states.

Delocalisation speeds up S 2 reactions.

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Effect on Actual Rates

Actual Rates are Affected by Substrate and Conditions

Nucleophiles

S 2accelerated by good nucleophile (Nu in the rate step)

S 1unaffected by nucleophile (Nu not in the slow step) 

Good Nu favours S 2

Poor Nu slows S 2 

Leaving Groups

S 2accelerated by good leaving group (LG in the rate step)

S 1accelerated by good leaving group (LG in the slow step) 

Good LG favours S 2 and S 1

Poor LG slows S 2 and slows S 1 (even more)

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What Makes A Good Nucleophile

An available lone pair of electrons - often an anion but also a loosely held lone pair.

In general nucleophilicity increases down the periodic table (lone pairs more 

polarisable due to shielding of nucleus by inner e- shells)

SH2 > OH2                                            I- > Br- > Cl-

and decreases across the periodic table (more electronegative atoms hold the 

lone pair more tightly)

NH3 > H2O

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Nucleophile League Table

Nucleophile                               Rel. Rate

I-, HS-, RS-                                  >10                               v. good

Br-, HO-, RO-, CN-, N -               10                                  good

NH , Cl-, F-, RCO -                     10                                   ---

H O, ROH                                    1                                     poor

RCO H                                        10-                                 v. poor

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What Makes A Good Leaving Group?

A stable molecule is forced out but often it is an anion.

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Leaving group ability of halides follows conjugate acid strengths.

                                     F-   <<   Cl-   <   Br-   <   I-

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Leaving Group League Table

                      Leaving group               Rel. Rate                    pKa conj. Acid

poor                     HO-                              slow                             15 

                             F-                               10-^5                            3.5

                            Cl-                               10^-1                             -7

good                    Br-                                 1                                 -9

                             I-                                 100                              -10

                          H2O                                10                          -1.7  (H3O+)

                         TsO-                                10^5                           -2.8

very good        CF SO -                           10^8                              -6

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pKa and Ka Equations

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Questions

1. Why must a different system be considered for tertiary alkyl halides?

2. The more stable the cation intermediate the _______er the mechanism.

3. In what two ways can cations be stabilised?

4. Name some features of S 1 reactions.

5. How does resonance stabilise carbocations?

6. How do adjacent lone pairs stabilise carbocations?

7. Is there a reaction by the S 1 system at primary centres?

8. What are the S 1 and S 2 reactions each activated by in benzene?

9. What happens when we mix two orbitals?

10. What does delocalisation so to the speed of S 2 reactions?

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More Questions

11. What is the effect of a good nucleophile on S 2 reactions?

12. What is the effect of a good nucleophile on S 1 reactions?

13. What is the effect of a good leaving group on S 1 and S 2 reactions?

14. Name some factors which make a good nucleophile.

15. Name two very good nucleophiles and one very bad one.

16. What does the leaving group ability of halides follow?

17. Name two good leaving groups and two poor ones.

18. What is the equation used to calculate pKa

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