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