Alkenes Flashcards
Alkene structure
Unsaturated hydrocarbons
Cn H2n, branched alkenes
No general formula for cyclic and many double bonds
Double bond nature
Carbon atoms in a double bond have 3 sigma bonds and 1 pi bond
Pi bond, formed by 2 lengthway overlap of p orbitals
Locks 2 carbons, no rotation
Shape
Trigonal planar
3 electron dense areas in same plane
Stereoisomers
Same structural formula, different arrangement of atoms in space
E/Z isomerism, C=C
Optical isomers, wide range of compounds
E/Z isomers
Rotation about double bond is restricted, carbon fixed relative to each other due to rigid pi bond
C=C
Different groups attached to each carbon of double bond
Same side, Z
Different side, E
Cis-trans isomerism
Special case of E/Z isomerism
Attached groups to each carbon must be hydrogen
Cahn-Ingold-Prelog rules
Priority based on atomic no
Group with higher atomic no at first point of difference=higher
Reactivity of alkenes
More reactive than alkanes due to pi bond
Pi electrons exposed than sigma electrons, breaks more readily, addition reactions
Pi bond enthrall is weaker than sigma bond
Addition reactions of alkenes
Hydrogen with nickel catalyst
Halogens
Hydrogen halides
Steam in presence of an acid catalyst
Hydrogenation of alkenes, elec addition
Passed over nickel catalyst, form alkane
Halogenation of alkenes
Test for saturation
Bromine added across double bond, orange to colourless
Haloalkanes
Alkenes and hydrogen halides
Alkenes and gaseous hydrogen halides at room temp
If alkene a gas, reaction takes place when gases are mixed
Form haloalkane
Hydration reactions
Alkenes and steam in the presence of H3PO3 acid catalyst
Alcohol
Electrophilic addition reactions
Pi electrons attract electrophiles
Arrow must start at bond/lone pair
Arrow head to atom
Electrophile
Molecule that is attracted to electron rich centres, accepts electron pair
