Alkenes

HideShow resource information
  • Created by: Hindleyc
  • Created on: 17-05-18 17:44
What are alkenes?
Unsaturated hydrocarbons
1 of 70
What do they contain?
Atleast one carbon-carbon double bond- a centre of high electron density
2 of 70
What do this make alkenes in comparison to alkanes?
More reactive than alkanes as high electron density of c=c leads to attack on these molecules by electrophiles
3 of 70
General formula?
CnH2n
4 of 70
What is the arrangement of bonds around the C=C? Angle?
Planar, 120
5 of 70
How do you test for alkenes?
Add bromine water and if double bond present de-colourises it
6 of 70
How can it be used quantitatively?
To show presence of multiple double bonds in compounds eg polyunsaturated
7 of 70
What does the double bond consist of?
One sigma bond and one Pi bond
8 of 70
Which one is exposed?
Pi
9 of 70
Which has high electron density?
Pi
10 of 70
What does this mean?
Vulnerable to attack by electrophiles- species which like electrons
11 of 70
Why are the electrons available to attack?
Further away
12 of 70
Which bond is weaker and further away and therefore more susceptible to attack than sigma?
Pi bond
13 of 70
Which bond is it that is broken when the double bond is broken?
Pi bond
14 of 70
What does the presence of the pi bond do?
Prevent rotation around sigma bond
15 of 70
What does this lack of rotation give rise to?
E-Z (geometrical) isomers
16 of 70
What is this type of isomerism an example of?
Stereoisomerism
17 of 70
What is a stereoisomer?
Have same structural formula but different spatial arrangement of atoms
18 of 70
What can a single c-c bond do?
Easily rotate
19 of 70
How do they arise?
Restricted rotation around c=c double bond. There are 2 different groups/atoms attached to both ends of the double bond
20 of 70
How do you name an EZ isomer
Determine priority group on both sides of the double bond (atom with bigger Ar)
21 of 70
How do split bond for EZ?
Vertically
22 of 70
When is it labelled Z?
When the priority atom is on the same side of the double bond
23 of 70
When is it labelled E?
If the priority atom is on the opposite side of the double bond
24 of 70
What reactions do alkenes undergo?
Electrophilic addition
25 of 70
What are the double bonds in alkenes?
Areas with high electron density
26 of 70
What does this do?
Attracts electrophile and undergo addition reactions
27 of 70
Whats an addition reaction?
Where two molecules react together to produce one
28 of 70
Whats an electrophile?
Electron pair acceptor
29 of 70
What are the electrophiles?
Bromine, hydrogenbromine and sulphuric acid
30 of 70
What is the change is functional group with Bromine? what is the reagent? Conditions? Mechanism? Type of reagent?
Alkene to dihalogenoalkane. Br2. Room temp. Electrophilic addition. Electrophile- Br slightly positive
31 of 70
How does it work?
As the Br2 molecule approaches the alkene, the pi bond electrons repel the electron pair in Br-Br bond inducing a dipole so Br2 becomes polar and electrophilic
32 of 70
Mechanism?
E- go from the double bond to the slightly +ve Br and form a bond, The e- in the Br2 molecule move to the slightly -ve Br atom forming a Br- and a carbocation. This negative ion is attracted to the positive charge on the carbocation and bonds
33 of 70
What is a carbocation?
The intermediate formed which has a positive charge on the C atom
34 of 70
Change is fg for Hydrogenbromide and alkenes? Reagent? Conditions? Mechanism? Type of reagent?
Alkene- halogenoalkane. HCl or HBr. Room temperature. Electrophilic addition. Electrophile- slightly positive H
35 of 70
How does it work?
HBr is a polar molecule because Br is more electronegative than hydrogen . The slightly positive hydrogen is attracted to the electron rich pi bond.
36 of 70
Mechanism?
E-s move from the double bond to the slightly +ve H atom and form a bond. The e-s then move from the h-br bond to the Br atom making it a negative ion and forming a carbocation. The br ion then bonds to the positive carbocation forming halogenalkane
37 of 70
What can the reaction lead to? Why?
2 Products when the alkene is unsymmetrical
38 of 70
Stage 1 Change is fg for Sulfuric acid and alkenes? Reagents? Conditions? Mechanism? Type of reagent?
Alkene to alkylhydrgoen sulphate. Concentrated H2SO4. Room temp. Electrophilic addition. Electrophile-H2SO4
39 of 70
Overall role of sulphuric acid? how?
Catalyst as it is regenerated at the end
40 of 70
Mechanism?
Electrons move from bond to slightly positive hydrogen, electrons in h-oso2oh bond move to oso2oh forming negative ion. Ion then bonds to carbocation forming CH3CH2SO4
41 of 70
What happens in stage 2?
The alkyl hydrogen sulphate in converted into an alcohol
42 of 70
What are the reagents?
Water
43 of 70
Conditions?
Warm mixture
44 of 70
Type of reaction?
Hydrolysis
45 of 70
What is hydrolysis?
Reaction where the molecule is split by the addition of water
46 of 70
What happens?
The OSO2OH turns into OH by the addition of water forming H2SO4 and alcohol
47 of 70
What is Markownikoffs rule?
In most cases, the bromine will be added to the carbon which has the fewest hydrogens attaches to it
48 of 70
If the alkene is unsymmetrical what can the addition of HBr lead to?
2 isomeric products
49 of 70
Why?
Wants to form most stable carbocation. It is more stable if methyl groups are on either side of positive carbon as they are electron releasing so reduce charge on ion= stabilises it
50 of 70
Order of stability for carbocation?
Tertiary>secondary>primary
51 of 70
What do they then form?
Major and minor products
52 of 70
What is the other reaction with alkenes?
Direct industrial hydration of alkenes to form alcohols
53 of 70
How?
Reacted with water in presence of acid catalyst in one step
54 of 70
Conditions? Temp? Pressure? Catalyst?
High- 300-600. High 70atm. Concentrated H3PO4
55 of 70
Why can't it be done in a lab?
High pressure needed.
56 of 70
Why is it preferred industrially?
No waste product and high atom economy. Separation of products easier and cheaper to carry out
57 of 70
eqn?
CH2=CH2(g)+H2O(g)=CH3CH2OH(l)
58 of 70
What are addition polymers with regards to society?
Commercially important and many uses in modern society
59 of 70
How are they formed? what is the process called?
From alkenes and substituted alkenes. Addition polymerisation
60 of 70
Reaction/eqn? what does n mean. What is only 1 repeating unit?
Alkene monomer eg ethene CH2=CH2 goes into a polymer [-CH2-CH2-]n poly(ethene). n=no. of repeating units. no n.
61 of 70
When do you add n?
if writing an equation showing the reaction where ‘n’ monomers become ‘n’ repeating units
62 of 70
What is poly(chloroethene)?
pvc- polymer that is waterproof- electrical insulator and doesn't react with acids
63 of 70
What is it like in its pure form?
Rigid plastic from the strong IM forces so dent move over each other (upvc)
64 of 70
What is upvc used in?
Windowframe coverings and gutterings
65 of 70
What happens if plasticisers are added?
The IM forces are weakened- which allows chains to move more easily meaning more flexibility
66 of 70
What is this used in?
insulation on electrical wires and waterproof clothing
67 of 70
Why are poly(alkenes) like alkanes unreactive?
Due to strong c-c and c-h bonds
68 of 70
What are addition polymer chains like? polarity? saturation- what does this mean?
HC chain long inc V.D.W and inc M.P and B.P. Non-polar so insoluble. Long and saturated- unreactive- can't react with electrophiles nucleophiles or addition reactions
69 of 70
What is recycled?
Poly(propene) is recycled
70 of 70

Other cards in this set

Card 2

Front

What do they contain?

Back

Atleast one carbon-carbon double bond- a centre of high electron density

Card 3

Front

What do this make alkenes in comparison to alkanes?

Back

Preview of the front of card 3

Card 4

Front

General formula?

Back

Preview of the front of card 4

Card 5

Front

What is the arrangement of bonds around the C=C? Angle?

Back

Preview of the front of card 5
View more cards

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Calculations resources »