WARNING!
Download
1 / 53

WARNING! This document contains visual aids for lectures It does not contain lecture notes It does not contain act - PowerPoint PPT Presentation


  • 268 Views
  • Uploaded on

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’

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'WARNING! This document contains visual aids for lectures It does not contain lecture notes It does not contain act' - Audrey


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Slide1 l.jpg

  • 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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

Chlorine also adds to alkene C=C bonds

1,2-Dichlorobutane

1-Butene


Slide6 l.jpg

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 l.jpg

  • 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 l.jpg

  • 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 l.jpg
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 l.jpg

  • 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 l.jpg
Aromatic systems in pharmaceuticals

atorvastatin

(Lipitor®)

sildenafil

(Viagra®)

miconazole


Slide16 l.jpg

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 l.jpg

Hybridisation

2e-

1e-

1e-

1e-

1e-


Slide18 l.jpg

  • 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 l.jpg

  • 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 l.jpg

  • 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 l.jpg

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 l.jpg

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 l.jpg

6-Methyl-3-octyne

1-Heptene-6-yne

4-Methyl-7-nonen-1-yne


Slide24 l.jpg

Linear geometry of alkynes difficult to accommodate in a cyclic structure

Hence relatively few cycloalkynes

Smallest stable cycloalkyne is cyclononyne

Cyclononyne


Slide25 l.jpg

Hydrogenation of alkynes cyclic structure

  • 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 l.jpg

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 l.jpg

Alkyne (poisoning)

Cis-alkene

3-Heptyne

Cis-3-heptene


Slide28 l.jpg

  • [Na, liq. NH3; or Li, liq. NH3]

  • This gives specifically Trans-alkenes

3-Heptyne

Trans-3-heptene


Slide29 l.jpg

Cis sodium or lithium metal in liquid ammonia-2-hexene

Trans-2-hexene


Slide30 l.jpg

Addition of bromine sodium or lithium metal in liquid ammonia(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 l.jpg

Hydration of 1-alkynes sodium or lithium metal in liquid ammonia

  • [Addition of water]

  • Requires catalysis by mercury (II) salts

1-Alkyne

Ketones

4-Methyl-1-hexyne

Ketone


Slide32 l.jpg

Review: quantifying acid strength: sodium or lithium metal in liquid ammoniapKa

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 l.jpg

Can define an equilibrium constant sodium or lithium metal in liquid ammoniaKa’

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


Slide34 l.jpg

  • 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 l.jpg

p sodium or lithium metal in liquid ammoniaKa

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 l.jpg

  • 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 l.jpg

HC Carbons bearing the negative charge ≡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 l.jpg

The process is general for 1-alkynes Carbons bearing the negative charge

Sodium acetylides

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

1-Pentyne

3-Methyl-1-butyne

Acetylide anions


Slide39 l.jpg

  • 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 l.jpg

2-Pentyne Carbons bearing the negative charge

2-Methyl-3-pentyne

Propyne

2-Butyne


Slide41 l.jpg

Recall: Carbons bearing the negative charge

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 l.jpg

  • Chlorination of alkanes proceeds by such mechanisms

Homolytic

cleavage

  • [Heterolytic cleavage: cleavage into ions]


Slide43 l.jpg

Methane (CH which…4)

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 l.jpg

Propagation

Initiation

Propagation


Slide45 l.jpg

Chlorination of alkanes other than methane which…

e.g. 2-Methylbutane

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

3o C-H

2o C-H

1o C-H



Slide47 l.jpg

  • 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 l.jpg

[1] [2] [3] [4] which…

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 l.jpg

This trend reflects the relative stabilities of the intermediate free radicals

more stable than

More stable than


Slide50 l.jpg

Propyl group

Two possibilities

1-Propyl (‘Propyl’)

2-Propyl or Isopropyl


Slide51 l.jpg

Butyl group between substitutents of the same number of Carbons

Four possibilities

1-Butyl (‘Butyl’)

2-Butyl

orsec-Butyl

(“secondary-Butyl”)

tert-Butyl

(“tertiary- Butyl”)

Isobutyl

[or 2-Methyl-2-propyl]


Slide52 l.jpg

Free-Radical Polymerization (of Alkenes) between substitutents of the same number of Carbons

Examples


Slide53 l.jpg

Free radical polymerization mechanism between substitutents of the same number of Carbons

Require a free radical initiator (In•)

Termination

SC slides now available on ChemWeb