4.1.3 Alkenes Flashcards
(41 cards)
why are alkenes unsaturated hydrocarbons
they have at least 1 C=C double bond
what is the formula for alkenes, and under what conditions is it valid
CnH2n
- if aliphatic
- if branched
- (not if cyclic or contain more than 1 C=C)
what is each C=C double bond made of
3 sigma bonds
1 pie bond
what is a pi bond
- the sideways overlap of 2 p-orbitals, one from each carbon atom of the C=C, and each contributing 1 electron, above and below the plane of C-atoms
where is electron density concentrated in a pi bond
concentrated above and below the line joining the nuclei of the bonded atoms
why can pi bonds not rotate
the double bond locks the atoms into position, so atoms cannot rotate
what shape forms around the C=C
trigonal planar
why is the C=C bond trignal planar
- 3 electron pair regions around C atom
- electron pair regions repel each other as far as possible
- 120 degree bond angle
- no lone pairs, so don’t have to worry
- all atoms in the same plane
what are stereoisomers, and how are they different to structural isomers
have the same molecular and structural formula, but a different arrangement of atoms in space
- structural = same molecular formula, different structural formula
why can E/Z isomers occur
the C=C double bond is fixed due to pi bond
what are the conditions of E/Z isomers
1) must have C=C
2) must have 2 different groups on each C atom in the double bond
what is difference between E and Z isomers
E= the groups are attached diagonally across double bond
Z= the groups are on the same side of the double bone (Zee Zame Zide)
what are cis/trans isomers
special type of E/Z isomer where one of the groups on each carbon atom in double bond must be a H
- trans = E
- cis = Z
what is the rule for assigning priority where there are more groups present
assign priority based upon highest atomic number, and if one group has more than one atom, read from left to write, comparing atoms
- do separate chains on each C=C separately
how do you determine if an isomer with 4 groups is an E/Z isomer
E= the groups with the highest priority are diagonally placed across the C=C
Z= the groups with highest priority are on the same side of the C=C
why are alkenes more reactive than alkanes
- due to presence of pi-bond
- means the electron density is concentrated above and below the plane of sigma bonds
- so outside of double bond, means that the pi-electrons are more exposed than sigma electrons
- pi bond readily breaks to undergo addition reactions
how can you determine the enthalpy of the pi bond alone
- subtract the enthalpy of the C=C to the equivalent C-C bond
what is an addition reaction in alkenes
- there is an addition of a small molecule across the double bond
- breaking the pi-bond and forming new bonds
what are the 4 addition reactions of alkenes
1) hydrogenation
2) halogenation
3) hydrogen halides
4) hydration
explain the hydrogenation of alkenes
- alkenes mix with hydrogen
- pass over NICKEL CATALYST at 423K
- breaks ALL C=C
- forms equivalent alkane
( consider balancing the H2 so ensure enough to attach to all C=C)
explain the halogenation of alkenes
- rapid addition reaction with
- CHLORINE/BROMINE
- at ROOM temperature
how can you test the saturation of alkenes
BROMINE WATER:
- bromine water is orange
- if bromine is added across double bond
- will turn clear
- if no C=C, will stay orange
explain the reaction between alkenes and hydrogen halides ( e.g. chloride and bromide)
- react with GASEOUS hydrogen halides
- or CONCENTRATED ACID equivalenT (solutions of halides in H2O)
- at ROOM TEMPERATURE
- form haloalkanes
- if liquid alkene, halide is bubble through
- if gaseous alkene, gases are mixed
why can the hydrogen halide reaction with alkenes form more than 1 molecule
- if unsymmetrical alkene reacts with asymmetrical molecule, will form 2 product
