1. 20.1Types of organic reactions
HL only
Syed Arshad Mushtaq
Chemistry Teacher
Australian School of Abu Dhabi

2. Nucleophilic substitution reactions

3. Learning outcomes Understand:
Nucleophilic unimolecular substitution reactions are represented by SN1 and nucleophilic bimolecular substitution reactions are represented by SN2.
Carbocation intermediates are formed in SN1 reactions whereas SN2 reactions involve a concerted reaction with a transition state.
The predominant mechanism for tertiary halogenoalkanes is SN1 and for primary halogenoalkanes it is SN2. Secondary halogenoalkanes react by both mechanisms.
The rate determining step (slow step) for SN1 reactions depends only on the concentration of the halogenoalkane, rate = k[R−X]. For SN2 reactions, rate = k[R−X][Nu]. SN2 is stereospecific and involves inversion of configuration at the carbon atom.
SN2 reactions are favoured by aprotic, polar solvents and SN1 reactions are favoured by protic, polar solvents

4. Learning outcomes Apply knowledge to:
Explain why a hydroxide ion is a better nucleophile than a water molecule.
Deduce the mechanism of the nucleophilic substitution reactions of halogenoalkanes with aqueous sodium hydroxide in terms of SN1 and SN2 reaction mechanisms.
Explain how the rate depends upon the identity of the leaving group (i.e. the halogen), whether the halogenoalkane is primary, secondary or tertiary and on the choice of solvent.
Outline the difference between protic and aprotic solvents.

5. Substitution Reactions
In a substitution reaction, one atom or group of atoms, takes the place of another in a molecule
Examples
CH3CH2Br + KCN  CH3CH2CN + KBr
(CH3)3CCl + NaOH  (CH3)3 COH + NaCl 5

6. Nucleophilic Substitution
A nucleophile is a molecule or ion that has a high electron density.
It is attracted to atoms in molecules with a lower electron density.
It may replace another group in an organic molecule.
The molecule to which the nucleophile is attracted is called the substrate
The group that the nucleophile replaces is called the leaving group
These reactions are known as nucleophilic substitutions. 6

7. Nucleophilic Substitution
One covalent bond is broken as a new covalent bond is formed
The general form for the reaction is
Nu: R-X  R-Nu X:
Nucleophile Substrate Product Leaving group 7

8. Nucleophilic Substitution
Nu: R-X  R-Nu X:
The bond to the leaving group is broken
The leaving group takes both electrons that formed the bond with it
The nucleophile provides the electrons to form the new bond
Nucleophile Substrate Product Leaving group 8

9. Nucleophilic Substitution
halogenoalkanes commonly undergo nucleolophilic substitution reactions. The nucleophile displaces the halide leaving group from the halogenoalkane.
There are two common ways for nucleophilic substitutions to occur. They are known as SN1 and SN2.
Nucleophile Substrate Product Leaving group 9

10. Examples of Nucleophilic Substitutions
Nucleophilic substitutions may be SN1 or SN2 10

11. Nucleophilic Substitution Bimolecular or SN2
A reaction is bimolecular when the rate depends on both the concentration of the substrate and the nucleophile.
SN2 mechanisms occur most readily with methyl compounds and primary haloalkanes 11

12. SN2 Mechanism The general form for an SN2 mechanism is shown above.
Nu:- = nucleophile 12

13. An Example of a SN2 Mechanism
The nucleophilic substitution of ethyl bromide is shown
above. This reaction occurs as a bimolecular reaction.
The rate of the reaction depends on both the concentration
of both the hydroxide ion and ethyl bromide
This is a one step process since both the nucleophile and the substrate must be in a rate determining step. 13

14. Nucleophilic Substitution Unimolecular or SN1
A unimolecular reaction occurs when the rate of reaction depends on the concentration of the substrate but not the nucleophile.
A unimolecular reaction is a two step process since the subtrate and the nucleophile cannot both appear in the rate determining step
SN1 mechanisms occur most readily with tertiary haloalkanes and some secondary haloalkanes. 14

15. SN1 Mechanism The general form for an SN1 mechanism is shown above.
Nu:- = nucleophile 15

16. SN1 Mechanism
The first step is the formation of the carbocation. It is the
slow step. The rate of the reaction depends only on the
concentration of the substrate. 16

17. Inversion of configuration
SN1 and SN2 Reactions
SN1
SN2
Rate
=k[RX]
=k[RX][Nuc:-]
Carbocation intermediate?
Yes No
Stereochemistry
mix
Inversion of configuration
Rearrangement
~H, ~ CH3 possible
No rearrangements 17

19. Aprotic polar solvent and protic solvent
POLAR PROTIC SOLVENTS (polar and ability to be H-bond donor)
have dipoles due to polar bonds
can H atoms that can be donated into a H-bond
examples are the more common solvents like H2O and ROH
remember basicity is also usually measured in water
anions will be solvated due to H-bonding, inhibiting their ability to function as Nu
POLAR APROTIC SOLVENTS (polar but no ability to be H-bond donor)have dipoles due to polar bonds
don't  have H atoms that can be donated into a H-bond
examples are acetone, acetonitrile
anions are not solvated and are "naked" and reaction is not inhibited

20. Overall
All nucleophiles will be more reactive in aprotic than protic solvents
Those species that were most strongly solvated in polar protic solvents will "gain" the most reactivity in  polar aprotic (e.g. F-).
Polar aprotic solvents are typically only used when a polar protic solvent  gives poor results due to having a weak Nu, (esp. F-, -CN, RCO2-)

21. HYDROXIDE – A BETTER NUCLEOPHILE
Nucleophiles are reactants that are electron rich so they are very attracted to electron deficient atoms.
Nucleophiles have a lone pair of electrons and may also carry a negative charge: H2O, OH-, NH3, CN-
The hydroxide ion is a stronger nucleophile than water because it carries a negative charge.

22. Electrophilic addition reactions

23. Learning outcomes Understand:
Electrophiles are electron-deficient species that can accept a pair of electrons from a nucleophile. All electrophiles are Lewis acids.
The major product in electrophilic addition reactions involving asymmetrical alkenes with hydrogen halides and interhalogens can be predicted using Markovnikov’s rule.
The formation of the major product can be explained by the relative stability of the possible intermediate carbocations in the reaction mechanism.

24. Learning outcomes Apply their knowledge to:
Deduce the mechanism of the electrophilic addition reactions of alkenes with hydrogen halides and with halogens and or interhalogens.

25. Addition Mechanisms
Electrophilic addition occurs in reactions involving containing carbon-carbon double bonds - the alkenes.
An electrophile is a molecule or ion that is attracted to electron-rich regions in other molecules or ions.
Because it is attracted to a negative region, an electrophile carries either a positive charge of a partial positive charge 25

26. Electrophilic Addition
Electrophilic addition occur in molecules where there are delocalized electrons. The electrophilic addition to alkenes takes the following general form: 26

27. ALKENES + HALOGENS (Ethene + bromine)
When ethene gas is bubbled through bromine, the brown color disappears as it forms 1,2-dibromoethane.
Remember that the color change shows presence of unsaturation.

28. The mechanism is as follows:
Bromine, a non-polar molecule, becomes polarized as it approaches the electron rich area of the alkene.
The electron rich area repels the electrons on the bromine molecule causing an area of positive charge and an area of negative charge.

29. The bromine atom nearest the alkene’s double bond gains a partial positive charge and acts as the electrophile.
The bromine molecule splits heterolytically forming Br+ and Br- and the initial attack on the ethene in which the pi bond breaks is carried out by Br+.
This first step is slow forming an unstable positive carbocation intermediate.

30. This carbocation then reacts rapidly with the negative bromide ion forming the product.

31. If you mix this reaction with the presence of chlorine ions, the carbocation readily reacts with either the Cl- or Br- ions.
However, no dichloro compound is ever formed confirming that the initial attack is from the Br+ ion.

32. ALKENES + HYDROGEN HALIDES (Ethene + hydrogen bromide)
The reaction occurs by a similar reaction as alkenes + halogens.
HBr is a polar molecule that undergoes heterolytic fission to form H+ and Br-.
The electrophile, H+, attacks the alkene’s double bond.
The unstable positive carbocation intermediate then reacts readily with the Br- to form the addition product.

33. A piece of evidence that supports this mechanism is that the reaction if favored by a polar solvent that facilitates the production of ions from heterolytic fission.
HI > HBr > HCl

34. Markovnikoff’s Rule
Actually there are two possible carbocations that could be formed. In may cases this would result in two possible products. However only one form is preferred
“Birds of a feather flock together!”
The hydrogen ion will tend to migrate to the side with the greater number of hydrogen atoms. This preference is known as Markovnikoffs Rule. 34

35. Example The electrophilic addition of alkenes occurs in two stages
First there is the formation of a carbocation
Followed by the attack the chloride ion to form the addition product 35

36. Sample Problem
Write a mechanism for the electrophilic addition of HBr to 1-butene. 36

37. Please note in picture partial charges on H-Br are not correct
Solution
Write a mechanism for the electrophilic addition of HBr to 1-butene.
Solution
Please note in picture partial charges on H-Br are not correct 37

38. ALKENES + INTERHALOGENS (Ethene + BrCl)
An interhalogen is a compound made up of two halogens.
Chlorine is more electronegative than bromine so BrCl is polarized so that Br has a partial positive charge and is the electrophile and Cl has the partial negative charge and will react with the carbocation.
Draw the reaction mechanism.

39. SAMPLE PROBLEM
Write the names and structures for the two possible products of the addition of the interhalogen BrCl to propene.
Which is likely to be the major product? Explain.

40. Solution

42. Electrophilic substitution reactions

43. Learning outcomes Understand:
The simplest arene (aromatic hydrocarbon compound) is benzene.
Benzene has a delocalized structure of π bonds around its six-membered ring.
Each carbon to carbon bond has a bond order of 1.5.
Benzene undergoes electrophilic substitution reactions.

44. Learning outcomes Apply knowledge to:
Deduce the mechanism for the nitration of benzene using concentrated nitric acid and a catalyst of concentrated sulfuric acid.

45. Orbital model of Benzene Structure
Benzene is built from hydrogen atoms (1s1) and carbon atoms (1s22s22px12py1).
Promotion of electron
Hybridization

46. sp2 hybrid orbital

47. sp2 hybrid orbital

48. Benzene undergoes electrophilic substitution reactions
Electrophilic substitution reactions involving positive ions
Benzene and electrophiles
Because of the delocalised electrons exposed above and below the plane of the rest of the molecule, benzene is obviously going to be highly attractive to electrophiles - species which seek after electron rich areas in other molecules.
The electrophile will either be a positive ion, or the slightly positive end of a polar molecule

49. The general mechanism
The first stage
The second stage

50. The electrophilic substitution reaction between benzene and nitric acid
The facts
Benzene is treated with a mixture of concentrated nitric acid and concentrated sulphuric acid at a temperature not exceeding 50°C. As temperature increases there is a greater chance of getting more than one nitro group, -NO2, substituted onto the ring.
Nitrobenzene is formed.

51. The electrophilic substitution reaction between benzene and nitric acid
The formation of the electrophile
The electrophile is the "nitronium ion" or the "nitryl cation", NO2+. This is formed by reaction between the nitric acid and the sulphuric acid

52. The electrophilic substitution reaction between benzene and nitric acid
The electrophilic substitution mechanism
Stage one
Stage two

53. Reduction Reactions

54. Learning outcomes Understand:
Aldehydes can be reduced to primary alcohols.
Carboxylic acids can be reduced first to aldehydes then to primary alcohols.
Ketones can be reduced to secondary alcohols.
Typical reducing agents are sodium borohydride, NaBH4, and lithium aluminium hydride, LiAlH4, (used to reduce carboxylic acids

55. Learning outcomes Apply knowledge to:
Write equations for the reduction reactions of aldehydes to primary alcohols, ketones to secondary alcohols and carboxylic acids to aldehydes, using suitable reducing agents.

56. REDUCTION OF ALDEHYDES AND KETONES
Reducing agents
lithium tetrahydridoaluminate(III) (also known as lithium aluminium hydride)  LiAlH4
Sodium tetrahydridoborate(III) (sodium borohydride). NaBH4

57. Reducing agents LiAlH4 NaBH4
Both reducing agents produce the hydride ion, H-, which acts as a nucleophile on the electron deficient carbonyl carbon.
LiAlH4
NaBH4
Lithium aluminum hydride, LiAlH4, in anhydrous conditions, such as dry ether followed by aqueous acid.
LiAlH4 have violent reaction and only used for reduction of carboxylic acid
Sodium borohydride, NaBH4 in aqueous or alcoholic solution
NaBH4 is the safer reagent but is not reactive enough to reduce carboxylic acids

58. REDUCTION OF ALDEHYDES AND KETONES
The overall reactions
The reduction of an aldehyde
Reduction of an aldehyde leads to a primary alcohol.
The reduction of a ketone
Reduction of a ketone leads to a secondary alcohol

59. Reduction of carboxylic acid
Carboxylic acids are first reduced to aldehydes and than to primary alcohols

60. Nitrobenzene to Penylamine
Reduction Reaction

61. Learning out comes Application and skills
Describe the two-stage conversion of nitrobenzene to phenylamine

62. Nitrobenzene to phenylamine
The conversion is done in two main stages:
Stage 1: conversion of nitrobenzene into phenylammonium ions
Nitrobenzene is reduced to phenylammonium ions using a mixture of tin and concentrated hydrochloric acid.

63. Nitrobenzene to phenylamine
Stage 2: conversion of the phenylammonium ions into phenylamine
Sodium hydroxide solution is added to the product of the first stage of the reaction.

64. Summary
Benzene to Phenylamine

65. Chemguide.Uk