Alkenes

Question 1

what is the general formula of alkenes?

Answer 1

CnH2n

Question 2

what is a sigma bond?

Answer 2

a single covalent bind is a sigma bond. it is formed when two orbitals overlap, in a straight line, in the space between two atoms. this gives the highest possible electron density between the two + nuclei meaning that there is a strong electrostatic attraction between the nuclei and the shared pair of electrons - sigma bonds therefore have a high enthalpy

Question 3

what is a pi bond?

Answer 3

it is formed when two lobes of two orbitals overlap. it has got two parts to it, one above and one below the molecular axis (p orbitals can form pi bonds)
in a pi bond the electron density is spread out above and below the nuclei causing the electrostatic attraction between the nuclei and the shared pair of electrons to be weaker than in sigma bonds

Question 4

what type of binds does a double bind have and what does this mean?

Answer 4

it has both sigma and pi binds meaning that it is less than twice as strong as a single bond

Question 5

why can’t double binds rotate?

Answer 5

because of the way that the P orbitals overlap to for a pi bond. also they are double bonds and so they are quite rigid - things can still rotate about any single bonds in the molecule - this restricted movement is what causes alkanes to form stereoisomers

Question 6

what is E/Z isomerism?

Answer 6

a type of stereoisomerism - one of the groups is called a E-isomer and the other is called a Z-isomer, the Z-isomer has the same groups either both above or both below the double bond and the other has the same groups across the double bond

Question 7

what is a stereoisomer?

Answer 7

has the same structural formula but a different arrangement in space - because of the lack of rotation some alkenes can have these- they occur when the two double bonds have different atoms or groups attached to them

Question 8

how does the E/Z system work even when the groups are different?

Answer 8

atoms with a larger atomic number are given priority - you may have to look further along the chain if the atoms directly bonded to the carbon are the same to work out which one is of a higher priority.

Question 9

what is cis-trans isomerism?

Answer 9

it is a special type of isomerism. if the carbon atoms have at least one group in common then you can call it cis or trans isomerism
cis - means that the same groups are on the same side of the double bond and trans means that the bonds are on opposite sides of the bond

Question 10

what is an electrophilic addiction reaction?

Answer 10

in an electrophilic addition reaction, the alkene double bind opens up and atoms are added to the carbon atoms.
they happen because the carbons have lots of electrons and so are easily attracted by electrophiles - electron pair acceptors

Question 11

what does adding hydrogen to a c=c bond do?

Answer 11

it produces and alkane. ethene will react in an addition reaction to produce ethane, requires a nickel catalyst and a temperature of 150

Question 12

what do halogens react with hydrocarbons to form?

Answer 12

dihalogenoalkanes, the halogens add across the double bond. it is an electrophilic addition reaction

Question 13

when you shake an alkene with bromine water what happens and why?

Answer 13

the solution quickly decolourises as the bromine is added across the double bond to form a dihalogenoalkane

Question 14

how are alcohols made by steam hydration?

Answer 14

Alkenes can be hydrated by steam at 300 oC and a pressure of 60-70 atm. it needs a solid phosphoric(v) acid catalyst

Question 15

what are alkenes oxidised by?

Answer 15

acidified potassium manganate (VII)
if you shake the mixture the purple solution is decolourised - you have oxidised the alkene and made a diol (an alcohol with two OH groups)

Question 16

what is the product between an Alkene and a hydrogen halide?

Answer 16

it is an addition reaction which produces a halogenoalkane

Question 17

what are the two possible products that can be formed if the hydrogen halide adds to an unsymmetrical alkene?

Answer 17

the amount of each product depends on how stable the carbocation formed in the middle of the reaction is. carbocations with more alkyl groups are more stable because the alkyl groups feed electrons towards the positive charge
the major product from the addition of hydrogen halide is the one where hydrogen adds to the carbon with the most hydrogens already attached (Markownikoff’s rule)

Question 18

what do alkenes join to form addition polymers?

Answer 18

they form addition polymers. this addition is called addition polymerisation. to find the monomer used to form an addition polymer, take the repeat unit, add a double bond to the carbon atoms and remove the bond from each end.

Question 19

what are the different methods for disposing of polymers?

Answer 19

waste plastic can be buried
it can be reused
it can be burned

Question 20

describe the process of burying plastic?

Answer 20

landfill is used to dispose of waste when plastic is:
difficult to separate from otherwise separate form other waste
not in sufficient quantities to make separation financially worthwhile
too difficult technically to recycle

Question 21

describe the process of burning plastic?

Answer 21

if recycling is not possible then the plastic can be burned - heat from this can be used to generate electricity. this process needs to be carefully controlled to reduce toxic gases. waste gases are passed through scrubbers which can neutralise gases which can neutralise gases, such as HCL, by allowing them to react with a base.

Question 22

how are polymers made sustainably?

Answer 22

they can use reactant molecules that are as safe and environmental friendly as possible.
use as few other materials, like solvents, as kissable. if you have to use other chemicals use ones that won’t harm the environment.
Energy ixd should be kept to a minimum.

Question 23

when do biodegradable polymers decompose?

Answer 23

they are made of renewable materials such as starch, or from oil fractions such as the hydrocarbon isoprene