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slide1

WARNING!

  • This document contains visual aids for lectures
  • It does not contain lecture notes
  • It does not contain actual lectures
  • Failure to attend lectures can harm your performance in module assessment
  • Printing out handouts of PowerPoint documents
  • From ‘File’ menu, select ‘Print’
  • Set ‘Print range’ to ‘All’; set ‘Print what:’ to ‘Handouts’
  • Set ‘Slides per page’ to ‘3’ (recommended to facilitate taking of notes), ‘4’ or ‘6’
  • Click on ‘OK’
slide2

Addition of bromine (Br2) to alkenes

General reaction

  • Alkene p bond lost; two new C-Br s bonds formed
  • Stereospecific reaction observed with cycloalkenes

Cyclopentene

Trans-1,2-dibromo-

cyclopentane

(no cis-isomer)

slide3

Reaction mechanism involves two steps

1st Step: alkene p electrons attack Bromine

Bromide ion and a cyclic epibromonium ion results

  • The large size of Bromine w.r.t Carbon (4th row vs. 2nd row) means that it can span two Carbons

rather than

slide4

2nd Step: addition of bromide anion

Anion approaches epibromonium ion from the face opposite that blocked by bromine

With cyclopentene

Epibromonium ion

and bromide

Trans-1,2-dibromo-

cyclopentane

slide5

Chlorine also adds to alkene C=C bonds

1,2-Dichlorobutane

1-Butene

slide6

Molecular formula C6H6

Benzene

  • All Carbons and Hydrogens equivalent

Kekulé structure (1865)

=

  • However, does not behave like a typical alkene
  • Less reactive than typical alkenes
  • Only reacts with bromine in presence of a catalyst
  • A substitution rather than an addition reaction occurs

not

slide8

Also, all benzene C-C bond lengths equal: 139 pm

  • Comparison: C-C 154 pm; C=C 134 pm
  • Planar ring of sp2 hybridised Carbons
  • 6 pz orbitals overlap to form a continuous cyclic p system

p electron density located above and below the plane of the ring

  • 6 p electrons
  • All 6 C-C bonds equivalent
  • [Not a representation of benzene p molecular orbitals]
slide10

Arrangement of 6 p electrons in a closed cyclic p systems is especially stable

  • Said to possess aromaticity
  • Aromatic systems very common (e.g. benzene and its derivatives)

Representing the p system in benzene

  • Represents p system well
  • Of limited use in describing reactivity
  • Better to use a combination of Kekulé structures
some points about this representation
Some points about this representation
  • Neither Kekulé structure alone is an adequate representation of the p bonding in benzene.
  • An adequate representation requires both structures simultaneously
  • The structures are known as resonance forms or resonance contributors
  • Each resonance structure contributes [equally] to the overall p bonding system
  • ‘↔’ is used to show that structures are resonance forms of each other;
  • resonance structures are enclosed in square brackets
slide12

These are NOT independent species existing in equilibrium

  • The p electrons in benzene are said to be resonance delocalised over the entire ring system
  • Resonance delocalisation is generally energetically favourable
  • Resonance delocalisation of 6 p electrons in a closed ring system is especially favourable: aromaticity
aromatic systems in pharmaceuticals
Aromatic systems in pharmaceuticals

atorvastatin

(Lipitor®)

sildenafil

(Viagra®)

miconazole

slide16

Alkynes

Older name: Acetylenes

  • Characterised by the presence of Carbon-Carbon triple bonds
  • General structure of alkynes
  • Groups R, C, C and R are co-linear
  • Neither sp3 nor sp2 hybridised Carbon consistent with this geometry
slide17

Hybridisation

2e-

1e-

1e-

1e-

1e-

slide18

Two sp hybridised orbitals can be arrayed to give linear geometry

  • Two remaining 2p orbitals are mutually orthogonal and orthogonal to the two sp hybridised orbitals
  • [If the two sp orbitals lies along the z axis, 2px lies along the x axis and 2py along the y axis]
slide19

Overlap of sp orbitals on two Carbons results in s bond formation

s

=

  • [s* also formed; not occupied by electrons]
  • px orbitals overlap to form a p bond in the xz plane

p

[p* also formed;

not occupied]

  • py orbitals overlap to form a p bond in the yz plane

p

[p* also formed;

not occupied]

slide20

C≡C consists of one s bond and two p bonds

  • The s bond lies along the C-C bond axis
  • The bond axis lies along the intersection of orthogonal planes
  • One p bond lies in each plane, with a node along the bond axis

View along the bond axis

slide21

A triple bond consists of the end-on overlap of two sp-hybrid orbitals to form a σ bond and the lateral overlap of the two sets of parallel oriented p orbitals to form two mutually perpendicular π bonds

slide22

First two members of the series of alkynes

Ethyne

(Acetylene)

Propyne

Nomenclature

  • Prefix indicates number of carbons (‘eth…’, ‘prop…’, etc.)
  • Suffix ‘…yne’ indicates presence of C≡C

Butyne

Can have C≡C between C1 and C2

or between C2 and C3

1-Butyne

2-Butyne

  • These are structural isomers
slide23

6-Methyl-3-octyne

1-Heptene-6-yne

4-Methyl-7-nonen-1-yne

slide24

Linear geometry of alkynes difficult to accommodate in a cyclic structure

Hence relatively few cycloalkynes

Smallest stable cycloalkyne is cyclononyne

Cyclononyne

slide25

Hydrogenation of alkynes

  • Standard hydrogenation conditions completely remove the p bonds
  • Both p bonds lost; four new C-Hs bonds formed

Heptane

3-Heptyne

  • [Conversion of alkyne to alkane]
slide26

Possible to modify the catalyst so as to reduce its activity (poisoning)

Lindlar’s catalyst

Pd/PbO/CaCO3

  • Pd: catalytic metal
  • PbO: poison
  • CaCO3: supporting material
  • Hydrogenation of alkynes using Lindlar’s catalyst removes only one p bond
  • [Only two Hydrogens added to C≡C; products are alkenes]
  • Reaction occurs on catalyst surface; both Hydrogens added to same face of alkyne
  • Specifically Cis-alkenes produced
slide27

Alkyne

Cis-alkene

3-Heptyne

Cis-3-heptene

slide28

Alkynes can also be converted into alkenes by reaction with sodium or lithium metal in liquid ammonia

  • [Na, liq. NH3; or Li, liq. NH3]
  • This gives specifically Trans-alkenes

3-Heptyne

Trans-3-heptene

slide29

Cis-2-hexene

Trans-2-hexene

slide30

Addition of bromine (Br2) to alkynes

  • Can have addition to one or both alkyne p bonds

Alkyne

Trans-1,2-dibromo-

alkene

1,1,2,2-tetra-

bromoalkane

1,1,2,2-Tetrabromoethane

Ethyne

(Acetylene)

Trans-1,2-dibromo-

1-butene

1-Butyne

slide31

Hydration of 1-alkynes

  • [Addition of water]
  • Requires catalysis by mercury (II) salts

1-Alkyne

Ketones

4-Methyl-1-hexyne

Ketone

slide32

Review: quantifying acid strength: pKa

Conjugate

base

Acid

Proton

  • Extent of dissociation is medium dependent; hence medium should be defined
  • If not otherwise stated, assume medium is water

Acid

Base

Conjugate

acid

Conjugate

base

slide33

Can define an equilibrium constant Ka’

  • Assume concentration of water stays constant; remove [H2O] term to give the dissociation constant Ka
slide34

The stronger the acid HA, the greater the dissociation

  • The stronger the acid,the greater the value of Ka
  • Range of Ka values is vast; inconvenient numbers
  • For convenience, take logs; define:

pKa = - log10Ka

  • Stronger acid; greater Ka; smaller pKa
  • Weaker acid; smaller Ka; greater pKa
  • ‘Strong acid’: HCl pKa = -7.0
  • ‘Weak acid’: CH3CO2H pKa = 4.76
slide35

pKa

50.0

44.0

25.0

Conjugate

bases

  • Ethane and ethene are effectively devoid of acidity
  • Ethyne dissociates to a miniscule extent
  • Reflects the relative stability of the conjugate bases

Least stable

Most stable

slide36

Order of stability is related to the hybridisation of the Carbons bearing the negative charge

  • Increasing s character assists in stabilising negative charge on Carbon
  • s orbitals locate the excess electron density closer to the positively charged nucleus
  • By comparison, p orbitals have nodal points at the nucleus

s

p

slide37

HC≡CH pKa25

  • Extent of dissociation almost negligible
  • However, dissociation can be driven to completion by reaction with very strong base

Sodium amide

(Sodamide)

Sodium

acetylide

  • This reaction goes entirely to completion
slide38

The process is general for 1-alkynes

Sodium acetylides

  • Reaction of1-alkynes with sodium amide gives complete conversion into sodium acetylides

1-Pentyne

3-Methyl-1-butyne

Acetylide anions

slide39

Acetylide anions are strong Carbon nucleophiles

  • React with Carbon electrophiles to form new Carbon-Carbon bonds

Chloride anion

displaced

Acetylide anion attacks

methyl Carbon

Chloromethane

New C-C bond formed

slide40

2-Pentyne

2-Methyl-3-pentyne

Propyne

2-Butyne

slide41

Recall:

Etc.

  • Reaction mechanisms so far have involved nucleophiles reacting with electrophiles…
  • …and ionic intermediates
  • Covalent bond formation the occurs as a result of movement of pairs of electrons
  • Such mechanisms are known as polar mechanisms
slide42

New covalent bonds can also be formed by processes in which…

  • …each molecular species involved donates one electron
  • Chlorination of alkanes proceeds by such mechanisms

Homolytic

cleavage

  • [Heterolytic cleavage: cleavage into ions]
slide43

Methane (CH4)

Methyl radical

  • Methyl radical is a neutral species bearing an unpaired electron
  • Is said to be a ‘free radical’
  • Methyl radical can react with further chlorine molecules
  • This step generates product and further chlorine atom
slide44

Overall process is a chain reaction

Propagation

Initiation

Propagation

slide45

Chlorination of alkanes other than methane

e.g. 2-Methylbutane

  • Substrate contains primary (1o), secondary (2o) and tertiary (3o) Hydrogens

3o C-H

2o C-H

1o C-H

slide47

Four products obtained in unequal amounts

  • If all Hydrogens on the substrate were equally reactive towards chlorine atom, would expect:

[1] [2] [3] [4]

50% 25% 17% 8%

Based on

Expected ratio [1]:[2]:[3]:[4] = 6:3:2:1

slide48

[1] [2] [3] [4]

34% 16% 28% 22%

Observed ratio of products

  • Less of products [1] and [2] than expected
  • More of product [3] than expected
  • Substantially more of product [4] than expected

Conclusion: Hydrogens not all equally reactive towards chlorine

Relative reactivity

most reactive 3o > 2o > 1o least reactive

slide49

This trend reflects the relative stabilities of the intermediate free radicals

more stable than

More stable than

slide50

Primary, secondary, tertiary system used to distinguish between substitutents of the same number of Carbons

Propyl group

Two possibilities

1-Propyl (‘Propyl’)

2-Propyl or Isopropyl

slide51

Butyl group

Four possibilities

1-Butyl (‘Butyl’)

2-Butyl

orsec-Butyl

(“secondary-Butyl”)

tert-Butyl

(“tertiary- Butyl”)

Isobutyl

[or 2-Methyl-2-propyl]

slide53

Free radical polymerization mechanism

Require a free radical initiator (In•)

Termination

SC slides now available on ChemWeb

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