alkenes

Question 1

what are alkenes

Answer 1

alkenes are unsaturated hydrocarbon

they are made up of hydrogen and have one or more carbon carbon double bonds

Question 2

what does the double bond in alkenes do to the number of hydrogens in alkenes

Answer 2

it mean that alkenes have fewer than maximum possible number of hydrogen atoms

Question 3

what does the double bond do to the reactivity of alkenes

Answer 3

the double bond makes alkenes more reactive than alkanes because of the high concentration of electrons (high electron density)between the two carbon atoms

Question 4

what are alkenes used for

Answer 4

ethene is a starting material for a large range of products

• the starting material for a range of products e.g.
  polymers such as polyethene e.t.c
• paints, antifreeze

Question 5

what is the general formula for alkenes

Answer 5

CnH2n

Question 6

what is the unique nature of alkenes

Answer 6

unlike the C-C bonds in alkanes, there is no rotation about the double bond

this is because of the make - up of a double. Any molecules in which a hydrogen atom in ethene is replaced by another atom or group will have the same flat shape around the carbon - carbon double bond

Question 7

why can’t the double bond cannot rotate

Answer 7

as well as a normal C-C single bond, there is a p - orbital

(which contains a single electron)

these 2 orbitals overlap to form an orbital with a cloud of electron density above and below the single bond

This Is called a pi orbital and its presence means the bond cannot rotate.
This is sometimes called restricted rotation

Question 8

what are the two types of isomers that involve the double bond

Answer 8

• position isomers

- geometrical isomers

Question 9

what are the position isomers

Answer 9

they are isomers where the double bond is in different positions

the longer the carbon - carbon, the more possibilities there will be and therefore the greater the number of isomers

Question 10

what are geometrical isomers

Answer 10

it is a form of stereoisomerism

the two stereoisomers have the same structural formula but the bonds are arranged differntly in space

it occurs only around the C=C bond

e.g. but-2-ene can exists as:
z-but-2-ene

E-but-2-ene

Question 11

what does the E-Z rotation based off of

Answer 11

the E-Z rotation is based on atomic numbers

HOW DO WE DO THIS:
-look at the atoms attached to each of the carbon atoms in the double bond
when the two atoms (of each pair) of a higher atomic number are on the same side of the C=C, the isomer is described as Z

the other isomer has the position of the Cl and hydrogen reversed
This is described as an E isomer

Question 12

what are the physical properties of alkenes

Answer 12

the double bond does not greatly affect properties such as boiling and melting points

• van der Waals forces are the only intermolecular forces that act between the alkene molecules
  This means that thee physical properties of the alkenes are very similar to those of the alkenes
• melting and boiling points increase with the number of carbon atoms present
• alkenes are not soluble in water
• are non - polar because the electronegativity of carbon and hydrogen are similar - hence why the intermolecular forces between their molecules are (weak) van der Waals forces

Question 13

how do alkenes react

Answer 13

the double bond makes a big difference to the reactivity of the alkenes compared with alkanes

The bond enthalpy for C=C is 612kJ mol–1 so you might predict that alkenes would be less reactive than alkanes
HOWEVER, ALKENES ARE MORE REACTIVE THAN ALKENES

The C=C forms an electron-rich area in the molecules, which can easily be attacked by positively charged reagents are called electrophiles (electron liking)

Question 14

what are some examples of good electrophiles

Answer 14

an example of a good electropile (electron liking)is the H+ ion

Question 15

what are most reactions of alkenes

Answer 15

As alkenes are unsaturated they can undergo addition reactions
In conclusion, most of the reactions of alkenes are electrophiles

Question 16

do alkenes burn in the air

Answer 16

alkenes will burn in the air

However, they are not used as fuels. This is because their reactivity makes them very useful for other purposes

Question 17

what electrons are used in electrophilic substitution

Answer 17

reactions of alkenes ar typically electrophilic addition.

The four electrons in the carbon- carbon double bond make it a centre of high electron density

electrophiles can form a bond by using two of the four electrons in the carbon carbon double bond (of the four electrons, the two are in a pi bond)

Question 18

what is the mechanism of electrophilic substitution

Answer 18

1. the electrophile is attracted to the double bond
2. electrophiles are positively charged and accept a pair of electrons from the double bond. The electrophile may be a positively charged ion or have positively area
3. a positive ion ( a carbocation) is formed
4. a negatively charged ion forms a bond with the carbocation

Question 19

how do alkenes react with hydrogen halides (an electrophile

Answer 19

hydrogen halides e.g. HCl , HBr and HI add across the double bond to form a haloalkane

e. g. imagine HBr reacting with alkenes
1. Bromine is more electronegative than hydrogen, so the hydrogen bromide molecule is polar H+ — Br-
2. the electrophile is the H+ of the H+of the HBr
3. One of the pairs of the electrons from C=C forms a bond with the H+ to form a positive ion (called a carbocation), whilst at the same time, the electrons in the HBr bond are drawn towards the BR-
4. The bond in hydrogen bromide breaks hetrolytically. Both electrons from the shared pair in the bond go to the bromine atom because it is more electronegative than hydrogen leaving a Br-ion
5. The Br-ion attaches to the carbocation forming a bond with one of its electron pairs

Question 20

what happens when the double bond is not exactly in the middle

Answer 20

when hydrogen bromide adds to the ethene,bromoethane is the only possible product

However, when the double bond is not exactly in the middle of the chain, there are two possible products-the bromine of the hydrogen bromide could bond to either of the carbon atoms of the double bond

Question 21

what are the two products of the electrophilic substitution of propene with HBr

Answer 21

it is 2-bromopropane / 1-bromopropane

however, the main product is entirely 2-bromopropane

Question 22

why is 2- bromopropane the main product while

Answer 22

alkyl groups e.g. CH3 or C2H5 for example, have a tendency to release electrons

This is known as a positive inductive effect and is sometimes represented by an arrow along their bond to show the direction of the release

This electron releasing effect tends to stabilise the positive charge of the intermediate carbocation
Essentially, the more alkyl groups there are attached to the positively charged atom, the more stable the cation is

Question 23

which alkanes are more stable tertiary, primary or secondary

Answer 23

so, a positively charged carbon atom which has three alkyl groups called a tertiary carbocation is more stable than one with two alkyl groups (secondary carbocation) which is more than one with just one (primary carbocation)

tertiary> secondary > primary

Question 24

how are alkenes converted to alcohols

Answer 24

they are converted to alcohols in a two-step reaction

first with an electrophilic addition reaction with H2S04 and the hydrolysis with water
H2SO4 is regenerated by the ned of the reactions an so it acts as a catalyst

Question 25

how are alkenes converted to alcohols in the industry

Answer 25

industrially alkenes are converted to alcohols in a one-step rather than the two-step reaction with sulfuric acid

They are reacted with water in the presence of an acid catalyst

The reaction can be called hydration which is when water is added to a molecule

Question 26

what are the conditions for the hydration of alkenes to form alcohols

Answer 26

HIGH TEMPERATURE 300 to 600 C

HIGH PRESSURES 70atm

CATALYST OF CONCENTRATED H3PO4

the high pressures needed mean this cannot be done in the laboratory
It is preferred industrially, however as there are no waste products, the reaction has a high atom economy

It would also mean separation of products is easier and cheaper to carry out

Question 27

what are the conditions required for the reaction of alkenes with a halogen to form a dihydrogenalkane

Answer 27

REAGENT: Bromine

CONDITIONS: Room temperature (not in UV light)

MECHANISM: Electrophilic Addition

TYPE OF REAGENT: ELECTROPHILE, BRδ+

Question 28

what are the conditions required for the reaction of a halogen halide with an alkene to form a halogenoalkane

Answer 28

REAGENT: HCL or HBR

CONDITIONS: Room temperature

MECHANISM: Electrophilic Addition

TYPE Of REAGENT: Electrophile, Hδ+

Question 29

what type of polymers are formed from alkenes

Answer 29

Addition polymers are formed from alkenes

This is called addition polymerisation

Question 30

how do you draw a repeating unit

Answer 30

do not add the n to the diagram because the n represents a large number

Question 31

what is the reactivity of polyalkenes

Answer 31

polyalkenes are unreactive due to the strong C-C and C-H bonds

Question 32

what is poly(chloroethene)

Answer 32

it is a polymer that is a waterproof, and electrical insulator and does not react with acid

In its pure form, it is a rigid plastic due to the strong intermolecular bonding between polymer chains preventing them from moving over each other.

In this unplasticised form, it is used to make uPVC window frame covering and guttering

Question 33

what happens if a plasticiser is added to poly(chloroethene)

Answer 33

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