Chapter 13 - Alkenes

alkene

unsaturated hydrocarbon containing a C=C double bond comprising a pi bond and a sigma bond

general formula

CnH2n

π-bond

formed by the sideways overlap of adjacent p-orbitals above and below the bonding C atoms - creates areas of electron density

-locks carbon atoms so double bond has restricted rotation

why are π-bonds weaker than σ-bonds?

the sideways overlap of the p-orbitals has a smaller orbital overlap than the direct/head-on overlap of orbitals in the sigma bond so less energy required

why are alkenes very reactive?

the pi bond has a relatively low bonding enthalpy so it can readily break as its electrons are very exposed (high electron density)

what is the shape around the double bond?

trigonal planar = 120

stereoisomers

compounds with the same structural formula but with a different arrangement in space

Cahn-Ingold-Prelog (CIP) rules

the priority is given to atoms or groups with the highest atomic number to decide if E or Z stereoisomerism

E-Z isomerism

isomers resulting from different spatial arrangements around a double bond

-what is needed: restricted rotation of groups/atoms around double bond, different groups attached to each carbon atom

-the E or Z go at the beginning of name

E-isomer

when the atom/group with the highest atomic number on each carbon atom are on opposite sides (Enemies)

Z-isomer

when the atom/group with the highest atomic number of each carbon atom are on the same side (Zame Zide)

Cis-Trans Isomerism

when an atom/group attached to the carbon atom are the same

Cis-isomer = Z-isomer

Trans-isomer = E-isomer

electrophile

an electron pair acceptor

HYDROGENATION of alkenes

-reaction where hydrogen is added across double bond

-turns alkenes into alkanes

-conditions needed: temp = 150+, nickel catalyst

HYDRATION of alkenes

-reaction where steam is added across double bond

-turns alkenes into alcohols

-conditions needed: steam, high temp + pressure, acid catalyst (usually phosphoric acid)

electrophilic addition mechanism (DRAW)

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STABILITY of carbocations

electrophiles react with unsymmetrical alkenes so different products are formed with varying stability

-least to most stable = primary, secondary, tertiary

the inductive effect

when electrons from each alkyl group are pushed towards the positively charged carbon atom

-the larger the number of alkyl groups, the greater the inductive effect which causes increase in stability of carbocation
-helps to reduce positive charge of carbon atom + spreads it out

Markownikoff’s Rule

-electrophilic addition reactions with unsymmetrical alkenes will produce a major product and a minor product

-the more stable carbocation = major
-the less stable carbocation = minor

-order from minor to major = primary, secondary, tertiary

addition polymerisation

unsaturated alkenes converted into long saturated alkanes

-contains 1 monomer

general equation of addition polymerisation (DRAW)

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DISPOSAL of polymers -3 ways

1) recycling

2) combustion for energy production

3) use as an organic feedstock

advs + disadvs of combustion

advs = energy used to produce electricity

disadvs = formation of CO2, formation of HCl or Cl2, formation of CO

bioplastics

biodegradable = broken down by microorganisms into water, carbon dioxide + biological compounds

photodegradable = bonds broken down chemically by absorbing light