4.1.3 Alkenes
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what type of hydrocarbons are alkenes
• unsaturated > contain at least 1 carbon to carbon double bond
how is a pi bond formed
• by the sideways overlap of adjacent p-orbitals above and below the bonding C atoms
what is the nature of the double bond in an alkene
• contains a sigma + pi bond > pi bond locks the 2 Carbon atoms in position, preventing them from rotating around the double bond
what is the shape + bond angle around a double bond in alkenes
• trigonal planar • 3 regions repel each other as far as possible, so bond angle is 120
what is the general formula of an alkene
CnH2n
why are alkenes more reactive than alkanes
• contain pi bond > pi electrons are more exposed than electrons in sigma bond due to being on the outside > pi bond readily breaks + alkenes undergo addition reactions relatively quick
what are stereoisomers
• compounds with the same structural formula but with a different arrangement in space
what conditions are needed for E/Z isomerism
• a C=C double bond • diff groups attached to each carbon atom of the double bond
how can you use the Cahn-Ingold-Prelog (CIP) rules to identify E/Z stereoisomers
• atoms attached to each carbon atom in a double bond are given priority based on their atomic number • higher priority groups are used > if on same side of double bond then Z isomer > if diagonally places across double bond then E isomer
what is cis-trans isomerism
• a type of E/Z stereoisomerism where two of the groups attached to each carbon atom of the C=C group are the same
name the addition reactions of alkenes
• hydrogen in presence of nickel catalyst • halogens • hydrogen halides • steam in presence of an acid catalyst
what do each of the addition reactions of alkenes involve generally
• addition of a small molecule across the double bond causes the pi bond to break and new bonds to form
what are the conditions required for hydrogenation of alkenes
• hydrogen • nickel catalyst • 423 K
what happens in the hydrogenation of alkenes
• hydrogen added across the double bond forming an alkane alkene + H2 = alkane
what are the conditions required for the halogenation of alkenes
• rapid addition • aqueous halogen • room temp
what happens in the halogenation of alkenes
• halogen added across the double bond, causing it to open + halogen added
how can you test for the unsaturation of alkenes
• bromine water (orange) turns colourless in presence of a C=C bond > bromine adds across to the double bond
what conditions are required for the addition reactions of alkenes with hydrogen halides
• gaseous hydrogen halide • room temp • if alkene gas, then both gases mixed • if alkene liquid, hydrogen halide bubble through it > can also react with concentrated hydrochloric acid or concentrated hydrobromic acid (solutions of hydrogen halides in water)
what happens in the addition reactions of alkenes with hydrogen halides
• hydrogen + halogen add to the alkene forming a haloalkane • can form 2 products - major + minor
what are the conditions needed for hydration of alkenes
• steam (H2O) • acid catalyst - phosphoric acid (H3PO4)
what happens in the hydration of alkenes
• steam adds across the double bond • produces an alcohol • can form 2 possible products - major + minor
alkenes usually take part in addition reactions to form saturated compounds, what is the mechanism for this reaction called
• electrophilic addition
what is an electrophile
• an electron pair acceptor > atom / group of atoms attracted to electron-rich centre + accepts an electron pair > usually a positive ion / molecule containing an atom with a partial positive charge
why do alkenes attract electrophiles
• double bond in alkenes represents region of high electron density because of pi bond • high electron density of pi electrons attracts electrophiles