DF6 - Alkenes

Question 1

What do unsaturated fats contain?

Answer 1

C=C bonds + are related to alkenes

Question 2

Which compounds are found to be very likely to lead up to a build-up of cholesterol? Cis or trans?

Answer 2

Trans fats

Question 3

How are alkenes distinguished from other hydrocarbons?

Answer 3

By the presence of a C=C double bond which implies they are unsaturated hydrocarbons

Question 4

Does the boiling points of alkenes increase or decrease as the number of C atoms increases?

Answer 4

The bp increases - just like alkanes

Question 5

What is the general formula of alkenes?

Answer 5

CnH2n

Question 6

How are the rules for naming alkenes different from naming alkanes?

Answer 6

The ending of alkenes is ‘ene’

Everything else is the same

Question 7

What the other unsaturated hydrocarbons?

Answer 7

• Cycloalkenes

* Dienes

Question 8

What is a diene?

Answer 8

An unsaturated hydrocarbon containing 2 C=C double bonds

Question 9

How many C=C double bonds do cycloalkenes have?

Answer 9

One C=C double bond

Question 10

What is a sigma bond?

Answer 10

A direct overlap of s-orbitals (sigma orbitals) and an area of increased electron density

Question 11

What does a C-C bond consist of?

Answer 11

The two electrons are arranged between atoms in an area of increased electron density called a sigma bond

Question 12

What does a C=C double bond consist of?

Answer 12

A sigma bond + a pi bond

Question 13

What is a pi bond?

Answer 13

One pi bond consists of two areas of negative charge, one above + one below the line of atoms

Question 14

Draw a diagram to represent the sigma + pi bonds in a C=C double bond

Answer 14

Sigma bond is in between the two areas of negative charge which make up the pi bond. The sigma bond is also in between the two C atoms in C=C double bond)

Question 15

What is an electrophile?

Answer 15

A positive ion, or molecule with a partial positive charge, that will be attracted to a negatively charged region + react by accepting a lone pair of electrons to form a covalent bond

Question 16

Describe and explain the double bond in ethene. What is attracted to the C=C bond and what will happen when they react?

Answer 16

• The 4 electrons in the double bond of ethene give the region between the the two C atoms a higher than normal density of negative charge
• Electrophiles will be attracted to this negatively charged region + they can react by accepting a pair of electrons from the C=C double bond

Question 17

What happens when ethene gas is bubbled through bromine? What is it a good test for?

Answer 17

The red-brown bromine becomes decolourised - this is a good general test for unsaturation in an organic compound

Question 18

What does lower case delta mean?

Answer 18

(In this case) small charge

Question 19

What is a dipole?

Answer 19

A molecule that has two different charges at opposite ends

Question 20

What does the curly arrow show?

Answer 20

Shows two electrons moving from one area to another

Question 21

What does it mean for a molecule to be polarised?

Answer 21

To have a balance of charges (neutral) but one side is negative and one side is positive (neg. + pos. charge in centre splits to opposite sides)

Question 22

What is a carbocation?

Answer 22

An ion with a positively-charged carbon atom.

Carbocations react very rapidly with anything that has electrons to share

Question 23

What is a mechanism?

Answer 23

An organic reaction mechanism represents the sequence of events in a reaction using the movement of electrons represented by curly arrows

Question 24

What is an addition reaction?

Answer 24

A reaction where two or more molecules react to form a single, larger molecule

Question 25

What is electrophilic addition?

Answer 25

An addition reaction involving the electrophile initially attacking

Question 26

What is the other test for alkenes? Explain how it works

Answer 26

• Shaking the alkene with bromine water
• Water molecules have lone pairs of electrons + can act as nucleophiles in competition with bromide ions
• If bromine water is dilute, there will be many more water molecules than bromide ions present + the bromoalcohol will be the main product of the reaction
• This does not affect what you see - the bromine water is still decolourised

Question 27

Describe the reaction of ethene with hydrogen bromide + draw the reaction with product

Answer 27

• Ethene reacts readily at room temp. with a solution of HBr in a polar solvent - this is another example of electrophilic addition
• Alkenes also react with gaseous HBr but ions are not involved + the mechanism involves a radical addition

Question 28

Describe ethene’s reaction with water + draw the reaction with product

Answer 28

• At a high temp. + in the presence of a catalyst (phosphoric acid adsorbed onto solid silica), ethene and water (as steam) undergo an addition reaction
• The process is used for the industrial manufacture of ethanol
• The overall reaction is addition of water across the double bond. The addition of water to an alkene is an example of a hydration reaction

Question 29

Describe how ethene can be converted to ethanol in a laboratory + draw the mechanism for this

Answer 29

• In the laboratory, ethene can be converted to ethanol by first adding concentrated sulfuric acid + then diluting with water

Question 30

Describe the reaction between ethene and hydrogen

Answer 30

• It is another addition reaction but it involves H atoms + takes place on the surface of the catalyst
• A catalyst is needed to help break the strong H-H bond + form H atoms that can react with the alkene
• If a platinum catalyst is used, the process takes place under standard lab. conditions
• Nickel is a cheaper but less efficient catalyst - it needs to be finely powdered + the gases needheating to approx. 150 degrees under 5 atm pressure for hydrogenation to occur
• This hydrogenation reaction is the reaction used to make the unsaturated fats + oils more saturated