alkenes

Question 1

What are alkenes?

Answer 1

alkenes are unsaturated hydrocarbons with the formula CnH2n

Question 2

what is the arrangement of bonds around the c-c double bond like?

Answer 2

• it is planar and has the bond angle 120 degrees

Question 3

What is a sigma bond?

Answer 3

• a covalent bond in alkenes formed by the front to end overlap of orbitals. the electron density is concentrated between the nuclei of the bonding atoms.

Question 4

What is a pi bond?

Answer 4

-a covalent bond formed by the side to side overlap of atomic orbitals involving p orbitals.
- this results in electron density concentrated above and below the internuclear axis between the two atoms.

Question 5

why do pi bonds make alkenes reactive?

Answer 5

• pi bonds are exposed and have high electron density
• they are therefore vulnerable to attack by electrophiles.

Question 6

what is a stereoisomer?

Answer 6

• stereoisomers are molecules with the same structural formula but a different spatial arrangement of atoms.
• alkenes exhibit E-Z stereoisomers

Question 7

when do E-Z stereoisomers arise?

Answer 7

• when there is restricted rotation around the C-C double bond
• there are two different groups attached to both end of the double bond

Question 8

how do you name E-Z stereoisomers?

Answer 8

• determine the priority groups on the side of the double bond. if the priority groups are on the the same side then its z if not its e.

Question 9

What is an electrophile?

Answer 9

An electron pair acceptor.

Question 10

what is an addition reaction?

Answer 10

• a reaction where two molecules react together to produce one.
• a pi bond is broken and two new sigma bonds are formed

Question 11

what happens in the electrophilic addition of bromine with alkenes?

Answer 11

• as the HBr nears the alkene, one of the carbon-carbon bonds breaks and the pair of electron attaches to the slightly positive H end of Hbr
• this is heterolytic fission.
• hbr breaks to form a bromide ion, and a carbocation is formed.
• then the bromide ion acts as a nucleophile and attacks the carbocation
• addition of hbr across the double bond.
• this forms a halogenoalkane

Question 12

what are the conditions needed for the electrophilic addition of hbr into an alkene?

Answer 12

• room temperature

Question 13

How does Br2, a non polar molecule act as an electrophile?

Answer 13

• the double bond polarises the bromine molecule , making the bromine closest to it partially positive and the furthest bromine partially negative as electrons in the pi bond repel the electron pair in the bromine bromine bond.
• aqueous
• used in test for unsaturation.

Question 14

what happens in the electrophilic addition of bromine onto propene?

Answer 14

• give 2 isomeric brominated compounds
• hbr is unsymmetrical and can add in two ways
• products ae not formed to the same extent
• problem doesn’t arise in Ethene because its symmetrical.

Question 15

what is Markovnikov’s rule?

Answer 15

“in electrophilic addition to alkenes the major product is formed via the more stable carbocation intermediate”

Question 16

why do major and minor products form?

Answer 16

• the more alkyl groups bonded to the carbocation the more stable the intermediate is.
• alkyl groups push electrons towards the positive carbocation groups, stabilising it.
• the more stable it is , the more likely it is to form.

Question 17

what is the order of stability for carbocations?

Answer 17

• tertiary> secondary > primary.

Question 18

what happens in the electrophilic substitution of h2so4 into an alkene?

Answer 18

• as the h2so4 nears the alkene the carbon carbon double bond breaks and the pair of electrons attaches to the slightly positive H end of h2 so4
• the breaks the c-c double bond leaving a carbocation and an negative oxygen ion as an intermediate
• the negative oxide ion attaches to the carbo cation intermediate

Question 19

what are the conditions needed for the electrophilic addition of h2so4?

Answer 19

• cold concentrated sulfuric acid

Question 20

How is the alkyl hydrogen sulfate used to make alcohol?

Answer 20

-this product can be made into ethanol + sulphuric acid by adding with water.
- sulfuric acid acts as a catalyst.

Question 21

What happens in the electrophilic addition with water and a strong acid (hydration of alkenes to form alcohols)

Answer 21

• electrons in the double bond are attracted to h+ in h30
• this breaks the double bond forming a carbocation intermediate and an oxide ion
• negative oxide ion is attracted to the carbocation
• CH2=CH2 (g)+ H2O(g) -> CH3CH2OH(l)

Question 22

what are the conditions of electrophilic addition with water and a strong catalyst?

Answer 22

• high temperature (300 -600)
• high pressure 70 atm
• catalyst of concentrated H3PO4

Question 23

how do you test for alkenes with bromine water?

Answer 23

• bromine water decolorises in the presence of a double bond this can be used as a test for the presence of a double bond in a molecule. it can be used quantitatively to show the presence of multiple double bonds in compounds like polyunsaturated oils.

Question 24

what is an addition polymer?

Answer 24

• a polymer formed by chain addition reactions between monomers that contain a double bond.
• this is called addition polymerisation

Question 25

how do you draw a single repeating unit of a polymer?

Answer 25

- draw out the monomer with groups of atoms arranged around the double bond - add the brackets

Question 26

how do you draw a polymer?

Answer 26

- draw a repeating unit then add 'n' to represent that it repeats

Question 27

what is a plasticiser? and what do it do when added to polychloroethane? (PVC)

Answer 27

- If a plasticiser is added the intermolecular forces are weakened which allows the chains to move more easily, resulting in more flexibility in the polymer. In this form PVC is used to make insulation on electrical wires, and waterproof clothing

Question 28

what is polychloroethene

Answer 28

Poly(chloroethene) is a polymer that is water proof, an electrical insulator and doesn’t react with acids. In its pure form it is a rigid plastic due to the strong intermolecular bonding between polymer chains prevents them moving over each other. In this un-plasticised form it is used make uPVC window frame coverings and guttering.