Biological ChemistryFIRST YEAR ORGANIC CHEMISTRY

Lecture Four

Alkenes and Alkynes

Convenor : Dr. Fawaz Aldabbagh

Mechanism

HI > HBr > HCl > HF is the order of activity

C C

H X

C C

H

X+Slow

C C

H X

C C

H

X

fast

C CH H

HH -bond

C C

H OSO2OH

C C

H

C C

H

O

S

O

O O H

OSO3H

Slow

Fast

Write a Mechanism for the acid catalysed hydration of a DB

Markovnikov’s Rule H atom adds to the carbon atom which already has the most H atoms

H2CHC CH3 H2C

HC CH3

H Br Markovnikov addition Product

H Br

Unsymmetrical Alkene

C C

H

H

H

CH3

H Br

C C

H

H

H

CH3

H

Slow

Br

C C

H

H

H

CH3

H Br

C CSlow

CH3

H

H

H

H

2o Carbocation prefered

1o Carbocation

3o > 2o > 1o

Carbocation stability

2-Bromopropaneis the main product

1-Bromopropane – little formed

The Slow Step is the Rate Determining Step

C

R

R

R3o CARBOCATION

CH3 CHBr CH3

CH3

H2CBrH2C

Bromination of DB – This is an Ionic Mechanism

1. Bromine molecule becomes polarised

3. Formation of Bromonium cation and Bromide anion

2. Bromine bond breaks heterolytically

4. Back-side nucleophilic attack – opening of three membered ring5. Stereospecific Product

Test for DB or TBDecloroization of Br2/CCl4

colourless

Contrast with reaction between Bromine and Alkanes

C C

Br

Br

C C

Br

Br+

Br

C C

Br

C C

Br

Br

Oxidation of Alkenes

1,2-Ethanediol(ethylene glycol)

H2C CH2KMnO4, -OH, H2O

OH OHH2C CH2 + MnO2

LEO Says GER

Oxidation Gain of oxygen and LEOMn (+7) to (+4) ------GER

Hydrogenation

C

CH

H

C H

C H

Pt or Pd - catalystsolvent, pressure

C

C

H

H2 X CH H

CH H

Pt - catalystsolvent, pressure

Example Pt ethanol, 1 atm

cyclohexene cyclohexane

Terpenes are volatile constituents of plant resins and essential oils

The number of carbons present is always a multiple of five

CH2

CH3H3C

Myrcene(isolated from bay oil)

Limonene(isolated oil of lemons or oranges)

-Pinene(oil of pine trees)

CH3

CH3H3C

OH

Geraniol(oil of roses and other flowers)

O Camphor(from camphor trees)

CH3

Menthol(from peppermint)

OH

Monoterpenes have 10 carbons

Sesquiterpenes have 15 carbons

Diterpenes have 20 carbons

Triterpenes have 30 carbons

CH2

CH3H3C

-Farnesene(coating of apples)

H3C

CH2

Carotene

Lycopene

Tetraterpenes have 40 carbons

Molecules which contain p-orbitals on an atom adjacent to a DBMolecules with delocalised -orbitals – are called

conjugated systemsUV-rays can cause electronic transitions (excitations) of valence electrons from bonding to anti-bonding orbitals. The greater the delocalisation or number of alternating DBs – the lower the energy of excitation, and lower energy visible rays are absorbed.

IR causes vibrations or rotations of covalent bonds-IR Spectroscopy

C C

H

H

H

H H

C C

H

H H

C C

H

H

H

CH3 H

C C

CH3

H H

C C

H

H

H

Cl

C C

F

F

F

F

C C

H H

C C

H

H HH

C C

H

H

ClH

C C

F F

FF

n

Polyethylene

n

Polypropylenepropylene

ethylene

vinyl chloride

n

n

n

n

Poly(vinylchloride), PVC

n

n

n

n

TeflonTetrafluoroethene

Styrene

Polystyrene

Polymers are large molecules containing many identical repeating units (100-1000000)

Polymerisation reaction is a repetition reaction which combines many small molecules of monomer (alkene) to form a polymer

Addition polymer is a polymer in which the monomer simply add together with no other products formed besides polymer

AlkynesH C C H H3C C C H

Ethyne(acetylene)

Propyne

180o

Sp3 hybridisation in Saturated Bonds (e.g. alkanes)Sp2 hybridisation in DB (e.g. alkenes)Sp hybridisation in TB (e.g. alkynes)

C(6) - 1s2, 2s2, 2px1, 2py

1, 2pz0

C(6) - 1s2, 2s1, 2px1, 2py

1, 2pz1

Hybridization

2sp 2bondsTwo -bonds (C-H) and (C-C)And Two -bonds between C-C per C atom

Linear Molecule

Therefore, a Triple bond consists of one -bond and two -bonds

The two -bonds are perpendicular to each other and form a cylinder of negative charge about the axis of the bond

sp-orbitals contain 50% s- and 50% p-characterFar less disperse than sp2, which is less disperse than sp3

The Carbon-Carbon bond is 1.2Ao shorter than C=C, which is 1.3Ao. C-H bond is also shorter than ethene, which is shorter than ethane, because in ethyne it is overlap between an sp orbital and a s-orbital of H to give the -bond.The bonding electrons reside closer to the C-nucleus, and so are held more tightly.

Electronegativity – Electron Attracting Ability of species

Fluorine is the most electronegative element, followed by Oxygen and Chlorine

C CH H + B C CH

ethanyl anionalkanyl anion

+ HB

Ethyne is acidic

C CH R

F C

H

H

H

Cl C

H

H

H

TB is readily attacked by electrophiles

C CH3C CH3 CH

H3CC

Br

CH3

trans-2-Bromobutene

CH

H3CC

Br

CH3

HBr

HBrH

CHH3C C

Br

Br

CH3

2,2-DibromobutaneMarkovnikov Rule

2-Butyne

Alkynes are high energy compounds

C CH H + 2.5 O2 2 CO2 + H2O

Welding Gas