Alkenes

Question 1

what is an alkene?

Answer 1

an unsaturated hydrocarbon that contains at least 1 carbon-carbon double bond in their molecular structure.

Question 2

what is the general formula of an alkene?

Answer 2

CnH2n

Question 3

what 2 types of bonds do alkenes consist of?

Answer 3

Sigma (σ) Bond: The sigma bond forms when two atomic orbitals overlap head-on. In the case of the carbon-carbon sigma bond in alkenes, the hybridized sp2 orbitals overlap end-to-end along the internuclear axis, resulting in a strong and stable bond.
Pi (π) Bond: The pi bond forms when two unhybridized p orbitals overlap side-by-side, with their electron density concentrated above and below the internuclear axis. In alkenes, the pi bond arises from the overlap of the unhybridized p orbitals of the carbon atoms adjacent to the sigma-bonded carbons. The pi bond provides additional stability to the molecule but is weaker than the sigma bond.

Question 4

how do these 2 covalent bonds affect the reactivity of alkenes?

Answer 4

alkenes are more reactive than alkanes as the Pi bond is significantly weak and susceptible to attack by electrophiles. these attack lead to various addition reactions.

Question 5

how do these 2 covalent bonds affect the physical properties of alkenes?

Answer 5

boiling point is lower than alkanes as the double bond introduces higher polarity within the molecule which generally allows bonds to be broken faster and lowers the boiling points of molecules.

Question 6

how does a double bond affect the isomerism of a molecule?

Answer 6

alkenes exhibit geometric isomerism due to restricted rotation around the carbon-carbon double bond. this leads to the existence of cis-trans isomers where different spatial arrangements of substituent groups are possible around the double bond. these geometric isomers can have different physical and chemical properties, such as boiling points, melting points, and reactivity, depending on their spatial arrangement.

Question 7

what are primary alkenes and what do you know about their stability?

Answer 7

primary alkenes where the carbon atom directly bonded to the double-bonded carbon (the carbon-carbon double bond) is also bonded to only one other carbon atom. In other words, in a primary alkene, the carbon-carbon double bond is located at the end of the carbon chain, primary alkenes are generally less stable than secondary or tertiary alkenes due to the absence of additional alkyl groups.

Question 8

what are secondary alkenes and what do you know about their stability?

Answer 8

secondary alkenes are organic compounds that contain a carbon-carbon double bond (C=C) where the doubly bonded carbons are each bonded to two other carbon atoms. this arrangement leads to a structure where the carbon-carbon double bond is located between two alkyl groups. in other words, one of the carbon atoms of the double bond is bonded to two other carbon atoms, while the other carbon atom of the double bond is also bonded to two other carbon atoms. these alkenes exhibit moderate stability.

Question 9

what are tertiary alkenes and what do you know about their stability?

Answer 9

tertiary alkenes have the carbon-carbon double bond located between two secondary carbon atoms, meaning that both carbon atoms directly bonded to the double-bonded carbons have two carbon atoms attached.
tertiary alkenes are the most stable among primary, secondary, and tertiary alkenes due to the higher degree of substitution around the double bond.

Question 10

state the reagent, conditions, catalyst, product and equation for DIRECT HYDRATION

Answer 10

reagent- steam
conditions- high pressure
catalyst- phosphoric acid
product- alcohol
equation- C2H4(g) + H2O(g) ↔️ C2H5OH(g)

Question 11

state the reagent, conditions, product and equation for HYDROGENATION

Answer 11

reagent- hydrogen
conditions- nickel catalyst
product- alkanes
equation- C2H4(g) + H2(g) → C2H6(g)

Question 12

what is addition polymerisation?

Answer 12

an alkene undergoes an addition reaction with itself. all atoms in the original are used to form polymers.

Question 13

what is PVC?

Answer 13

a rigid polymer, used to form pipes. joined by intermolecular bonds, permanent dipole-dipole and london dispersion forces. intermolecular forces allow chains to move over eachother.

Question 14

what is deducing repeat units?

Answer 14

a repeat unit is the smallest group of atoms that when connected one after the other make up the polymer chain.
it is represented by square brackets in the displayed and general formula

Question 15

how do you name polymers in terms of alkenes?

Answer 15

poly(x), where x is the alkene.

Question 16

how does the unreactivity of polymers affect the environment?

Answer 16

polymers are non-biodegradable and take up to hundreds of years to decompose when dumped in landfill sites, therefore cause the long-term pollution of the environment. burning the polymers results in harmful combustion products which again cause the pollution of the environment.

Question 17

what does their unreactivity also affect?

Answer 17

they are resistant to chemical attack.

Question 18

how can you test for unsaturation?

Answer 18

mix the hydrocarbon sample with bromine water. if the solution loses its orange colour, it indicates the presence of unsaturated hydrocarbons.

Question 19

why are addition polymers unreactive?

Answer 19

once polymerisation reaction is complete, the polymer chains are essentially inert because their structure does not contain any reactive functional groups. they can become reactive again after chemical modification under specific conditions and reagents.

Question 20

explain the nature of intermolecular forces between molecules of polyalkenes.

Answer 20

contain van der waal (or london dispersion forces) due to the non-polar nature of these polyalkenes. these arise from temporary fluctuations in electron density within the molecule, leading to weak attractions between molecules.

Question 21

what is electrophilic addition?

Answer 21

an electrophile adds to a Pi bond, typically a double or triple bond. resulting in the formation of a two new Sigma bonds.

Question 22

what is an electrophile?

Answer 22

an electron-deficient species.

Question 23

name some electrophiles.

Answer 23

Hydrogen ions (H+)
Halogens (Br2)
Sulfuric acid (H2SO4)
Halogen halides (HBr)

Question 24

what is formed in the first step of electrophilic addtion?

Answer 24

when the double bond is broken, a carbocation is made, with a positive charge.

Question 25

explain the trend of carbocations.

Answer 25

they carry varying stability. tertiary being the most stable, primary being the least. the more stable the carbocation, the more likely it is to form. therefore they will always be a major and minor product in addition reactions.

Question 26

what are the conditions needed for branched alkenes to make them undergo polymerisation?

Answer 26

high pressure and temperatures create polymers with weak intermolecular forces from branched alkenes.

Question 27

what are the conditions needed for straight alkenes to make them undergo polymerisation?

Answer 27

require lower temperature and pressure than branched alkenes to make polymers with strong intermolecular forces.

Question 28

what catalyst is needed for addition polymerisation?

Answer 28

Ziegler-Natta-TiCl4