Alkenes Module 4

Question 1

What are Alkenes?

Answer 1

Have general formula CnH2n

Must have at least one C=C bond

Question 2

How do you name alkenes?

Answer 2

In between longest C chain and “ene” ending add where a C=C bond occurs and have “,” between them and all numbers separated by 2 “-“

Question 3

What’s a sigma bond?

Answer 3

Forms when 2 s orbitals overlap in a straight line, this gives the highest possible electron density between the 2 nuclei, this is a single covalent bond
The high electron density between the nuclei means there is a strong electrostatic attraction between the nuclei and shared pair of electrons

Strongest type of covalent bond

Draw these by having 2 spheres with a dot in the middle for a nucleus, and draw them overlapping in a diablo shape

Question 4

What’s a pi bond?

Answer 4

Formed from the sideways overlap of 2 adjacent p orbitals
It’s bellow and above the molecular axis, because the pi orbitals that overlap are dumbbell shaped
Pi bonds are weaker than sigma bonds because electron density spread above and bellow the nuclei, so weaker electro static attraction between nuclei and shared pair of electrons, so break easier than sigma bonds

Draw this by having 2 dumbell shapes with curved lines connecting each top of the dumbell

Which forms 2 dots with a curved pi bond above and bellow

Question 5

Why are Alkenes more reactive than Alkanes?

Answer 5

Alkanes only have C-C and C-H sigma bonds, whislt alkenes also have C=C pi bonds
C=C has a higher electron density as there are 4 electrons, pi bonds stick out, so they are more likely to attract electrophiles or nucleophiles, also pi bonds break easier

Question 6

Why can’t C=C bonds rotate and what does this cause?

Answer 6

Can’t rotate due to the way the p orbitals overlap to form a pi bond
Causes stereoisomers

Question 7

When does stereoisomerism occur?

Answer 7

When the two double bonded carbon atoms have each have 2 different atoms or groups bonded to them

Question 8

What’s a Z isomer?

Answer 8

When the same atom/group with the priority are both above or both bellow on each side

Question 9

What’s an E isomer?

Answer 9

When the same atom/group with the priority are on diagonal sides

Question 10

In steroisomerism how do you determine which is the priority group?

Answer 10

On each side it’s the group with the highest Mr, if they are the same will have to look at next Carbon in the chain and find which one has the highest Mr now

Question 11

What’s a cis isomer?

Answer 11

If the same groups are on the same side on each side of the C=C bond (don’t have to be priority)

Question 12

What’s a trans isomer?

Answer 12

If the same groups are on different sides of the C=C bond (don’t have to be priority)

Question 13

What type of reaction happens in alkenes?

Answer 13

Electrophillic addition, because double bond has plenty of electrons, so electrophiles (electron pair acceptors) are easily attracted

Question 14

Examples of electrophiles?

Answer 14

H+, NO2+ and polar molecules

Question 15

How do you produce an Alkane from an Alkene?

Answer 15

Add hydrogen gas

Ni catalyst and temperature around 150 degrees

Question 16

What’s the mechanism of halogens being reacting with alkenes to form dihaloalkanes?

Answer 16

Draw the alkene eg ethane, and draw the halogen eg bromine molecule near the double bond, the closer bromine atom will have a delta positive charge, as electrons have been repelled away by double bond, making the other Br molecule delta negative

Draw curly arrow from double bond to Delta + Br
Draw curly arrow from bond between Br’s to the delta negative Br

Draw the double bond now split to a single bond, and one C has a Br molecule attached, the other C will have a positive charge, and draw the other Br molecule with a negative charge and a lone pair with a curly arrow to the positively charged carbon

Draw the final dihaloalkane

Question 17

How do you use Bromine to check for Carbon double bonds?

Answer 17

Shake substance with bromine water, if alkene present orange bromine water decolourises as it’s been added across the double bond to form a haloalkane

Question 18

How do you make ethanol from Ethene?

Answer 18

Add water as steam at 300 degrees
60-70 atm
Phosphoric acid catalyst

Question 19

How do you produce a haloalkane from an alkene?

Answer 19

Add a hydrogen halide

It’s an addition reaction

Question 20

What are the 2 products called when a hydrogen halide reacts with an unsymmetrical alkene?

Answer 20

The major and minor product?

Question 21

when a hydrogen halide reacts with an unsymmetrical alkene how do you know which is the major and minor product?

Answer 21

Major product is more stable so the halide ion will bind to the Carbon which is bonded directly to the most other carbons ( least amount of hydrogens)

Minor product is less stable, so the halide ion will bind to the Carbon which is bonded directly to less Carbons(more amount of Hydrogens)

Question 22

When Alkene join up to form polymers how do you draw the polymer, repeat unit, and monomer?

Answer 22

Polymer (add poly in front of alkene name), then draw the whole chain (alkene repeated with double bond broken)

Repeat unit- draw the bit that’s repeated, with bracket going over side bonds, and n= amount of repeats

Monomer= the alkene of the polymer

Question 23

What’s an advantage/disadvantage of polymers?

Answer 23

Not reactive so good at carrying, however also means very difficult to dispose of

Question 24

How do we get rid of polymers?

Answer 24

Landfill
Recycled
Burned- heat energy can be used to generate electricity, however produces toxic and greenhouse gases

Question 25

What are biodegradable polymers?

Answer 25

Polymers which decompose naturally in certain conditions, as organism can digest them

Question 26

How would you draw a pi and sigma bond together?

Answer 26

Have the carbon atoms with a spherical Sigma bond between them (hot dog)
Have the (bun) pi bonds above them, with 2 hydrogen s coming of each Carbon

Question 27

2 Ways exactly how plastics can be recycled?

Answer 27

Plastics can be melted and remoulded

Some plastics can be cracked into monomers, which can be used as organic feedstock to make more plastics