13 - Alkenes Flashcards
Define alkene
Unsaturated hydrocarbon containing a C=C bond comprising of a TT bond (sideways overlap of adjacent p-orbitals above & below bonding C atoms) and a sigma bond (overlap of orbitals directly between bonding atoms).
Restricted rotation of TT bond
General formula of alkene
C n H 2n
Define sigma bond
When 2 s orbitals overlap in a straight line – gives highest possible electron density between 2 nuclei. Single covalent bond.
High electron density means there is a strong electrostatic attraction between nuclei and shared pair of electrons. High bond enthalpy, strongest type of covalent bond
What is a pi bond
Sideways overlap of two adjacent p orbitals. 2 parts – 1 above and 1 below the molecular axis, bc p orbitals which overlap are dumbbell shaped.
Much weaker than sigma bond bc electron density spread out above and below nuclei. Electrostatic attraction between nuclei and shared pair of electrons is weaker, so TT bonds have a relatively low bond enthalpy
Structure & bonding in alkene
Trigonal planar
120 degrees
C=C unit is planar
- 3 regions of electron density around each C atom.
- 3 regions repel as far apart as possible, so 120 degree bond angle
- all atoms in same plane
Define stereoisomer
Compounds with the same structural formula but a different arrangement in space
E/Z isomerism
An example of stereoisomerism, in terms of restricted rotation about a double bond, and requirement for 2 different groups to be attached to each carbon atom of the C=C group
Cis-trans isomerism
A special case of E/Z isomerism in which 2 of the substituent groups attached to each carbon atom of the C=C group are the same
3 isomer vs Z isomerism
higher priority group on …
E = either side (from equator lol), entgegen Z = together, zusammen
Cis vs trans
Cis = same group on either side, E Trans = same group on same side, Z
Reactivity of Alkenes
- much more reactive than alkanes
- relatively low bond enthalpy
- due to presence of TT bond
- TT electrons more exposed than sigma electrons -> TT bond breaks easily -> Alkenes undergo reactions easily
Addition reaction of Alkenes with hydrogen
- “hydrogenation of alkenes”
- nickel catalyst
- 423K
- alkene + hydrogen -> alkane
-one H2 molecule per double bond in alkene
Addition reaction of Alkenes with halogens
- “halogenation of alkenes”
- rapid addition reactions with halogens chlorine / bromine @ RT
- alkene + halogen molecule -> dihaloalkane
Addition reaction of alkenes with hydrogen halides
- alkenes react with GASEOUS hydrogen halides @ RT to form haloalkanes
- if alkene = liquid, bubble hydrogen halide through it.
- alkenes can react with concentrated HCl
- can form diff products
(explored further in electrophilic addition of alkenes)
Addition reaction of alkenes with steam
- “hydration reactions of alkenes”
- alkene + steam -> alcohol
- phosphoric acid catalyst, H3 PO4
- steam ADDS ACROSS double bond
- widely used to produce ethanol from ethene
- 2 possible products
How to test for unsaturation?
- reaction of alkenes with bromine
- when bromine water (orange coloured) added dropwise to alkene sample, bromine ADDS ACROSS double bond.
decolourises -> indicate presence of double bond
Define electrophile
An electron pair acceptor
Electrophilic addition
It is the mechanism for the addition reaction that alkenes usually take part in to form saturated compounds.
- double bond in alkene represents high electron density (bc of presence of TT electrons)
- high electron density of TT electrons attracts electrophiles
- electrophile: (group of) atoms attracted to electron-rich centre and accents electron pair. Usually positive ion / partial positive charge atom in molecule
Define carbocation
Contains a positively charged carbon atom
Propene and bromine reaction mechanism
Bromine is nonpolar. When bromine approaches propene, induced dipole is formed.
The bromine atom closer to the C=C is positive, other bromine atom is negative
Markownikoff’s rule
- predict formation of a major organic product in addition reactions of H-X to unsymmetrical alkenes
- major product: product with highest carbocation stability.
Tertiary carbocation = most stable
Explain carbocation stability
- Linked to electron donating ability of alkyl groups.
- Each alkyl group donates + pushes electrons towards positive charge of carbocation
- positive charge spread over alkyl groups
- more alkyl groups = more positive charge spread out = more stable ion
- so in inc order of stability: primary (2 H, 1 R), secondary (1 H, 2 R), tertiary (0 H, 3 R)
