Biological Chemistry FIRST YEAR ORGANIC CHEMISTRY Lecture Four Alkenes and Alkynes Convenor : Dr. Fawaz Aldabbagh

Mechanism HI > HBr > HCl > HF is the order of activity  -bond

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 Unsymmetrical Alkene

2 o Carbocation prefered 1 o Carbocation 3 o > 2 o > 1 o Carbocation stability 2-Bromopropane is the main product 1-Bromopropane – little formed The Slow Step is the Rate Determining Step 3 o CARBOCATION

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 ring 5. Stereospecific Product Test for DB or TB Decloroization of Br 2 /CCl 4 colourless Contrast with reaction between Bromine and Alkanes

Oxidation of Alkenes 1,2-Ethanediol (ethylene glycol) LEO Says GER Oxidation Gain of oxygen and LEO Mn (+7) to (+4) GER Hydrogenation

Terpenes are volatile constituents of plant resins and essential oils The number of carbons present is always a multiple of five Monoterpenes have 10 carbons Sesquiterpenes have 15 carbons Diterpenes have 20 carbons Triterpenes have 30 carbons

Tetraterpenes have 40 carbons Molecules which contain p-orbitals on an atom adjacent to a DB Molecules with delocalised  -orbitals – are called conjugated systems UV-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

Polymers are large molecules containing many identical repeating units ( ) 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

Alkynes Sp 3 hybridisation in Saturated Bonds (e.g. alkanes) Sp 2 hybridisation in DB (e.g. alkenes) Sp hybridisation in TB (e.g. alkynes) Hybridization 2sp 2  bonds Two  -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-character Far less disperse than sp 2, which is less disperse than sp 3 The Carbon-Carbon bond is 1.2A o shorter than C=C, which is 1.3A o. 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 Ethyne is acidic

TB is readily attacked by electrophiles Alkynes are high energy compounds Welding Gas