Alkenes

Question 1

Akenes are…

Answer 1

Unsaturated hydrocarbons.

Question 2

Alkenes have the GENERAL FORMULA…

Answer 2

CnH2n. They’re just made of carbon and hydrogen atoms, so they’re HYDROCARBONS.

Question 3

Alkene molecules ALL have at least one…

Answer 3

C=C double covalent bond.

Molecules with C=C double bonds are UNSATURATED because they can make more bonds with extra atoms in ADDITION reactions.

Question 4

Because there are two pairs of electrons in the C=C double bond, it has a …

Answer 4

really HIGH ELECTRON DENSITY. This makes alkenes pretty reactive.

Question 5

Covalent bonds form when …

Answer 5

ATOMIC ORBITALS from different atoms, each containing a single electron, OVERLAP, causing the electrons to become SHARED.

Question 6

A bond forms because the…

Answer 6

nuclei of the atoms are attracted by ELECTROSTATIC ATTRACTION to the bonding electrons.

Question 7

The WAY that atomic orbitals overlap causes…

Answer 7

different TYPES of bonds to form.

Question 8

Single covalent bonds in organic molecules are…

Answer 8

SIGMA BONDS

Question 9

A sigma bond is formed when…

Answer 9

TWO ORBITALS OVERLAP, in a straight line, in the space between two atoms.

This gives the highest possible ELECTRON DENSITY between two positive nuclei.

Question 10

The HIGHEST ELECTRON DENSITY between the nuclei means…

Answer 10

There is a strong ELECTROSTATIC ATTRACTION between the nuclei and the shared pairs of electrons.

This means that sigma bonds have a high BOND ENTHALOPY - they’re the STRONGEST type of covalent bonds.

Question 11

A double bond is made up of a …

Answer 11

SIGMA BOND and a PI-BOND.

A PI-BOND is formed when TWO-LOBES of two orbitals OVERLAP SIDEWAYS. It’s got two part to it - one ‘above’ and one ‘below’ the molecular axis.

Question 12

In a pie-bond, the electron density is …

Answer 12

SPREAD OUT above and below the nuclei. This causes the electrostatic attraction between the nuclei and shared pair of electrons to be WEAKER in pie-bonds than in sigma-bonds, so pie pie-bonds have a RELATIVELY LOW BOND ENTHALOPY.

Question 13

This means that a double bond (pie bond + sigma bond) is less than…

Answer 13

twice as strong as a single bond (just a sigma bond)

Question 14

In akenes, the C-C and C-H bonds are all..

Answer 14

SIGMA BONDS.

The C=C BONDS in alkenes contain both a SIGMA bond and a PIE BOND.

Question 15

ALLKENES are much more…

Answer 15

Reactive than alkanes.

Question 16

Alkenes only contain C-C and C-H …

Answer 16

SIGMA BONDS which have a high bond enthalopy and so are difficult to break.

The bonds are also NON-POLAR so they dont attract NUCLEOPHILES or ELECTROPHILES.

This means Alkanes don’t react easily.

Question 17

Alkenes are MORE REACTIVE than…

Answer 17

Alkanes because the C=C bond contains both sigma-bond and a pie-bond.

Question 18

Why are C=C bond more susceptible to electrophilic attacks?

Answer 18

The C=C double bond contains four electrons so it has a HIGH ELECTRON density and the PIE BOND also sticks out above and below the rest of the molecule.

These two factors mean the PIE BOND is likely to be attacked by ELECTROPHILES.

Thhe LOW BOND ENTHALOPY of the PIE bond also contributes to the reactivity of the alkenes.

Question 19

How are alkenes useful at making petrochemicals?

Answer 19

Because the double bond’s so REACTIVE, alkenes are HANDY STARTING points for making other organic compounds and for making PETROCHEMICALS.

Question 20

Consider the hydrocarbons ethane and ethene.

a) Explain how the type of bonding differes in those two molecules

Answer 20

Ethane is an alkane, so has a single C-C bond made up of a SIGMA bond.

Ethene is an alkene, so has a C=C double bond made up of a sigma bond and pie bond.

Question 21

Consider the hydrocarbons ethane and ethene.

b) Explain which of these molecules are more reactive.

Answer 21

Ethene will be more reactive than ethane because the double bonds has a high electon density with a pie bond which sticks out above and below the plane of the molecule, so it attracts electrophiles. also beacuse the pie bond has a low bond enthalopy so is more easily broken than the C-C bond sigma bond in ethane.

Question 22

The C=C bond in ethene is made up of two different types of bond.

a) Give one similarity between these bonds

Answer 22

Both bonds form when two atomic orbitals overlap.

Question 23

The C=C bond in ethene is made up of two different types of bond.

b) Give one difference between these bonds.

Answer 23

Sigma bonds form when two atomic orbitals overlap directly between the nuclei, whereas Pi-bonds form when both lobes of two P-orbitals overlap side in.