Module 4 - Alkenes

Question 1

what are alkenes

Answer 1

-unsaturated hydrocarbons
-CnH2n

Question 2

describe how sigma bonds are formed in alkenes

Answer 2

-two sp2 orbitals (one from each carbon) overlap to form a single C-C bond called a sigma σ bond

Question 3

describe how pi bonds are formed in alkenes

Answer 3

-rotation can occur around a sigma bond
-the π bond is formed by sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule

Question 4

describe pi bonds

Answer 4

-π bonds are exposed and have high electron density
-they are therefore vulnerable to attack by electrophiles
-the π bond is weaker than the σ bond

Question 5

describe the structure of alkenes

Answer 5

-planar around the C=C
-bond angle 120 degrees

Question 6

what are stereoisomers

Answer 6

-they have the same structural formulae but have a different spatial arrangement of atoms

Question 7

when does E-Z stereoisomerism arise

Answer 7

1. there is restricted rotation around the C=C double bond
2. there are two different groups/atoms attached both ends of the double bond

Question 8

define a priority group

Answer 8

-the atom with the bigger atomic number is classed as the priority atom

Question 9

describe Z isomers

Answer 9

-the priority atom is on the same side of the double bond

Question 10

describe E isomers

Answer 10

-the priority atom is on the opposite side of the double bond

Question 11

describe the CIP priority rules

Answer 11

-the atom of higher atomic number is given priority
-f the atoms are the same, consider the atoms at distance 2 from the double bond

Question 12

describe cis-trans isomerism

Answer 12

-a special case of EZ isomerism in which two of the substituent groups are the same

Question 13

define an addition reaction

Answer 13

-a reaction where two molecules react together to produce one

Question 14

describe the conditions needed to convert an alkene to an alkane

Answer 14

-hydrogen reagent
-nickel catalyst

Question 15

describe the conditions needed to convert an alkene to a dihaloalkane

Answer 15

-bromine reagent, electrophile reagent (Brδ+)
-room temperature

Question 16

what is the type of bond fission when alkenes react with a halogen

Answer 16

-heterolytic

Question 17

explain how alkenes react with a halogen (eg bromine)

Answer 17

-as the Br2 molecule approaches the alkene, the pi bond electrons repel the electron pair in the Br-Br bond
-this induces a dipole
-Br2 becomes polar and electrophilic (Brδ+)
-the intermediate formed (carbocation)

Question 18

describe the conditions needed for an alkene to be converted to a haloalkane

Answer 18

-HCl or HBr reagent, electrophile, HS+
-room temperature

Question 19

explain how alkenes react with HCl/HBr

Answer 19

-HBr is a polar molecule because Br is more
electronegative than H
-he H δ +is attracted to the electron-rich pi bond
-this reaction can lead to two products when the alkene is unsymmetrical

Question 20

what is markownikoff’s rule

Answer 20

-in most cases, bromine will be added to the carbon with the fewest hydrogens attached to it

Question 21

define an electrophile

Answer 21

-an electron pair acceptor

Question 22

explain what makes a carbocation intermediate more stable

Answer 22

-the more stable carbocation is stabilised because the methyl groups on either (or one) side of the positive carbon are electron releasing and reduce the charge on the ion

Question 23

how is the major product formed in electrophilic addition

Answer 23

-via the more stable carbocation intermediate

Question 24

what is the order of stability for carbocations

Answer 24

-tertiary > secondary > primary

Question 25

describe the conditions needed for alkenes to form alcohols

Answer 25

-steam reagent -temperature 300-600 degrees -70 atm pressure -catalyst of concentrated H3PO4

Question 26

describe how alkenes form alcohols

Answer 26

-they are reacted with steam in the presence of an acid catalyst -high pressure means it is done industrially rather than in the lab

Question 27

advantages of the production of alcohols from alkenes

Answer 27

-high atom economy as no waste products -separation of products is easier and cheaper to carry out

Question 28

define a hydration reaction

Answer 28

- a reaction where water is added to a molecule

Question 29

what is addition polymerisation

Answer 29

-addition polymers being formed from alkenes

Question 30

reactivity of poly(alkenes)

Answer 30

-like alkanes -unreactive -due to strong C-C and C-H bonds

Question 31

how to draw the polymer repeating unit for any alkene

Answer 31

-draw out the monomer with groups of atoms arranged around the double bond -then identify the repeating unit

Question 32

describe the industrial importance of alkenes

Answer 32

-forming polymers from ethene based monomers -manufacture of margarine by catalytic hydrogenation of unsaturated vegetable oils

Question 33

what does hydrogenation of liquid vegetable oils do

Answer 33

-converts the double bonds to single bonds -increasing the melting point of the oil so it is harder and more solid -forming margarine

Question 34

how are waste polymers processed

Answer 34

-separation and recycling -feedstock for cracking -combustion for energy production

Question 35

describe how waste polymers are used for separation and recycling

Answer 35

-the waste is sorted into each different type of polymer -then each type can be recycled by melting and remoulding

Question 36

impact of waste polymers being used as feedstock for cracking

Answer 36

-allowing for the new production of plastics and other chemicals

Question 37

describe how waste polymers can be used in combustion

Answer 37

-can be incinerated and heat is released to generate electricity -combustion of halogenated plastics can lead to the formation of toxic, acidic waste products such as HCl

Question 38

describe how chemists can minimise the environmental damage of combustion forming toxic products

Answer 38

-by removing the HCl fumes formed from the combustion process

Question 39

what have chemists been developing to deal with waste polymers

Answer 39

-biodegradable polymers -compostable polymers -soluble polymers -photodegradable polymers

Question 40

which polymers are biodegradable

Answer 40

-polymers formed from isoprene (2-methyl-1,3- butadiene), maize and starch

Question 41

define a saturated hydrocarbon

Answer 41

-a hydrocarbon that contains only C-C single bonds

Question 42

define an unsaturated hydrocarbon

Answer 42

-a hydrocarbon that contains C=C double bonds or more, or includes benzene ring