M4 B3 Alkenes
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What is the general formula for alkenes?
C n H 2n
What is the general formula for (cyclic) alkenes?
C n H 2n-2
What does unsaturated mean?
Contains atleast one multiple carbon carbon (c=c) bond.
Name the types of bonds present in alkenes (2)
• Sigma • Pi
What is a sigma bond?
The overlap of two orbitals directly between the bonding atoms
What is a pi bond?
The sideways overlap of adjacent p-orbitals above and below the bonding carbon atoms, which restricts rotation.
What is the bond angle of a sigma bond?
109.5 degrees
What is the bond angle of a pi bond?
120 degrees
What is the shape around a sigma bond?
tetrahedral
What is the shape around a pi bond?
trigonal planar
Define ‘stereoisomers’
Compounds with the same structural formula, but with a different arrangement of atoms in space.
E/Z isomerism is a form of stereoisomerism. To have E/Z isomerism a molecule must have: (2)
• C=C double bond which restricts rotation • Each carbon of the carbon= carbon double bond must be attached to two different groups.
How do you figure out which molecule is the higher priority (out of the two molecules attached to the SAME carbon) in an alkene?
If the atomic number of A is larger than the atomic number of B, then A is the higher priority group of the two. (A and B are on the same side of the c=c bond)
How do you figure out if the molecule is E or Z isomerism (out of the two molecules attached to the OPPOSITE carbon) in an alkene?
• If the highest priority groups are BOTH above or below the c=c, then it is Z isomerism. • If they are opposite then they are known as E isomerism.
What if the atomic number is the same across the groups attached to the c=c?
If two atoms attached to the same side of the c=c have the same atomic number, then the atomic number of the atoms bonded to these atoms is compared.
What is cis/trans isomerism?
This is a special case of E/Z isomerism in which 2 of the substituent groups are the same. (The two groups which are not the highest priority).
Reactions of alkenes: Why are alkenes more reactive than alkanes?
Alkenes are more reactive because of the relatively low bond enthalpy of the pi bond. (this is due to the reduced overlap as the p-orbitals overlap side-on.
Reactions of alkenes are addition reactions, what will the atom economy be?
100%
FORMATION OF ALKANES - Name of reaction? - General equation? - Reagents? - Conditions? - Mechanism? - Industrial importance?
• Name of reaction= Hydrogenation • General equation= alkene+H2=Alkane • Reagents= alkene ans hydrogen • Conditions= Nickel catalyst, 150 degrees (c) • Mechanism= electrophilic addition • Industrial importance= Manufacture of margarine from unsaturated vegetable oils.
FORMATION OF DIHALOALKANES - Name of reaction? - General equation? - Reagents? - Conditions? - Mechanism? - Industrial importance?
• Name of reaction= bromination/ halogenation • General equation= alkene+halogen=dihaloalkane • Reagents= alkene and bromine (aqueous) • Conditions= Room temp (dropwise and shake) • Mechanism= electrophilic addition • Industrial importance= Tests for the presence of c=c bond, changes to colourless.
FORMATION OF HALOALKANES - Name of reaction? - General equation? - Reagents? - Conditions? - Mechanism? - Industrial importance?
• Name of reaction= Adding HBr (hydrogen halides) • General equation= alkene+HBr=haloalkane (mono) • Reagents= alkene and hydrogen halide • Conditions= room temperature • Mechanism= electrophilic addition • Industrial importance= manufacture of haloalkanes
FORMATION OF ALCOHOLS - Name of reaction? - General equation? - Reagents? - Conditions? - Mechanism? - Industrial importance?
• Name of reaction= Hydration (adding STEAM) • General equation= alkene+ steam= alcohol • Reagents= alkene and steam • Conditions= concentrated H3PO4 (aq), high temp and high pressure. • Mechanism= electrophilic addition • Industrial importance= Industrial formation of alcohol.
Hydrogen halides are unsymmetrical. If they react with unsymmetrical alkenes, how many products can be formed?
2 possible products are formed. (different placement of the added groups)
What is an electrophile?
An electrophile is a species that accepts a pair of electrons.