Alkenes Flashcards
(17 cards)
Describe structure and the formula of alkenes
An unsaturated hydrocarbons, have a double carbon bond somewhere in the structure
Formula= CnH2n
Arrangement of the C=C bond is planar and has bond angles of 120 degrees
Why are alkenes attacked by electrophiles?
Pi bonds are exposed in the carbon double bond of the alkene
Pi bonds have a high electron density, so they are vunerable to attack by species which like electrons, electrophiles
What is a sterioisomer?
Isomers with the same structural forumla but different spatial arrangement of atoms
Why do alkenes exhibit stereoisomerism?
There is restricted rotation around the C=C bond
There are two different groups/atoms attached at both ends of the CC bond
Why might an alkene not exhibit E/Z sterioisomerism?
When there are two identical groups attached to the end of the restricted double bond, E/Z isomerism does not occur
For example in But-1-ene there is no E/Z isomerism as there is two identical groups on the end of the restricted double bond (2 hydrogens on the carbon) whereas in but-2-ene there is e/z isomerism because there is two different groups attached at either end of the restricted double bond
Describe why a carbon double bond has a relativley high enthalpy compared to carbon carbon single bonds
-C=C forms an electron rich area in the molecule which can be easily attacked by positively charged reagents ( electrophiles)
-These are electron pair acceptors, for example the H+ ion
Describe the principle of electrophilic addition reactions
(basic)
-Electrophiles are attracted to the 4 electrons in the centre of the C=C bond, and can form a bond by using 2 of the electrons ( the ones in the pi bond)
Describe the electrophilic addition mechanism detailed
Ethene + HBr e.g
In a molecule there is usually a difference in electronegativity
For example in hydrogen bromide the Br has a higher electronegativity than the H, meaning the Br attracts the pair of electrons slightly towards it, meaning the H has a slight positive charge, this means the H is an electrophile
H is now attracted to the pair of electrons in the exposed pi bond in the C=C bond in an alkene( ethene)
This means the electrons in the pi bond are attracted to the S+ charge on the H
These approaching pi electrons repel electrons in the H-Br bond down towards the Br
H atom then joins the carbon using 2 electrons from the pi bond, the carbon atom next to the carbon bonded to the H gets a + charge, and Br has the electron pair which originally joined it to H forming a - ion
Electrons in the lone pair on the Br are strongly attracted to the = charge on that carbon, forming a daitive bond between the Br and the carbon
Describe the trend in electrophilic addition for the halogen haldies
All hydrogen halides add on to the alkene in the same way
Only difference is the rate at which this occurs, the rate inc as you go down group 7
This is because as you go down group 7 the atom size increases, meaning the bond strength of these atoms with hydrogen decreases
If the bond is weaker the hydrogen halide bond will break easier (readily) so the reaction is more likely to happen
Describe markownioffs rule
In most cases the halogen is added to the carbon with the fewest hydrogens attached to it
The hydrogen is added to the carbon with the most hydrogens attached to it
Only the hydrogens on either side of the double bond count
Describe how electrophilic addition is different with asymertical alkenes
Double bond is not exactly in the middle of the chain, the bromine of the hydrogen bromide could bond to either of the carbon atoms of the double bond
E.g propene could form either 1-bromopropane or 2-bromopropane
Product is most of the time 2 bromopropane
The alkyl groups which are attached to the positive carbon atom have an electron releasing tendency which stabilises the positive charge on the carbocation,the more stable the carbocation is the more likely its formed as a product
- Most stable is tertiary then secondary then primary
- Draw out carbocations , state which ones more stable
Describe the test done which tests for unsaturation of hydrocarbons
Alkene and bromine water in test tube
Shake well with bung on test tube
Bromine water decolourises
Yellow- colourless NOT CLEAR
For example reaction with ethene
C2H4 + Br2= CH2BrCH2Br
One br adds on to each C in the double bond
Describe what happens during addition polymerisation
Addition polymers are formed from alkenes
Double bond opens up between alkene monomers
The alkene is unsaturated and the polyalkene is saturated as there are no double carbon bonds in the hydrocarbon chain
Describe the reactivity of addition polymers
- Unreactive, there are no carbon double bonds in the hydrocarbon chain
C-C and C-H bonds are very strong like alkanes
Describe bp of addition polymers
High melting and boiling points because they are long molecules, alot of electrons, means they have alot of van der waals, require a high amount of energy to break
Describe how the properties of PVC- poly (chloroethene) can be modified using a plasticiser
Give uses of the PVC in this form
They make polymers more flexible as they weaken intermolecular forces which allows the chains to move around easier
Uses in this form are insulation of electrical wires and waterproof clothing
Describe the properties of the pure form of poly( chloroethene) - PVC
Give properties of the PVC in this form
Pure form is a ridgid plastic due to strong intermolecular bonding between polymer chains which prevents them moving over eachother
Uses in this form are upvc window frames, coverings and gutterings ( drains)
