OZ 6, 7, 8

Question 1

why are the properties of haloalkanes different to properties of normal alkanes

Answer 1

due to the polar carbon halogen bond

Question 2

define electronegativity

Answer 2

a measure of the ability of an atom in a molecule to attract electrons in a chemical bond to itself

Question 3

how does electronegativity affect haloalkanes

Answer 3

• carbon halogen bonds are polar, but not polar enough to make haloalkanes soluble, or to change the physical properties of haloalkanes

Question 4

how to halogens affect boiling point of haloalkanes

Answer 4

• the bigger the amount of halogens, the larger the boiling point
• and the larger the atom (iodine is the biggest) the larger the boiling point

Question 5

describe the trend of boiling points in alkanes

Answer 5

• the longer the alkane molecule, the higher the boiling point
• also if it is a straight chain molecule it is more likely to have a higher boiling point because the molecules are able to pack more closely together which allows more dipole bonds to form

Question 6

what does it mean when a molecule is polarised

Answer 6

It means that there is a dipole present due to its polar bonds

Question 7

when do permanent dipoles occur

Answer 7

• in a polar molecule
• when atoms have big electronegativity differences

Question 8

does CCl4 have dipole? explain

Answer 8

• no it does not have a dipole even though it is formed with polar bonds
• central carbon has a positive charge whilst chlorines have a negative charge
• the tetrahedral and symmetrical shape of the molecule allow the charges to cancel out
• so no overall dipole

Question 9

how do instantaneous dipoles arise

Answer 9

the electrons in the bond are constantly in motion and when they are unevenly distributed at one point in time, the end that they are closer to has an instantaneous negative dipole and the other side has a positive dipole

Question 10

how do induced dipoles arise

Answer 10

• they arise when a non polar molecule is close to a dipole
• the dipole in the other molecule attracts the shared electrons and induces a dipole
• this can be caused by a permanent or instantaneous dipole

Question 11

state and describe each of the three types of intermolecular bonds

Answer 11

INST D, IND D-
- the weakest
- they are in ALL molecules, because instantaneous dipoles can arise in molecules that have permanent ones
- the more electrons in the atom, the greater the dipole effect
PER D, PER D
- relatively strong
- these substances are more likely to be liquids or solids
PER D, IND D
- e.g. in HCl or chlorine molecules

Question 12

explain the trend in boiling and melting points in the halogens in terms of intermolecular bonding

Answer 12

• halogens form non polar diatomic molecules with themselves
• so the strongest type of bond that can form is instantaneous dipole induced dipole bonds
• and the more electrons in the atom, the stronger the intermolecular bonds
  -fluorine is the smallest, and has the least electrons so has the lowest melting and boiling points
• which increases as you go down the group

Question 13

what is hydrogen binding

Answer 13

• the strongest type of intermolecular bonding
• a special case of permanent dipole permanent dipole bonding

Question 14

what are the main three key factors about hydrogen bonding

Answer 14

• there must be a large dipole between a hydrogen atom and a highly electronegative atom (oxygen, nitrogen or fluorine)
• a small hydrogen atom which can get very close to the electronegative atom
• a lone pair of electrons must be on the oxygen, nitrogen or fluorine

Question 15

why is water so unique in terms of bonding

Answer 15

• because it can make twice as many hydrogen bonds as other substances
• oxygen as two lone pairs of electrons and two hydrogens present per oxygen

Question 16

when and why do liquids have a high viscosity

Answer 16

• when liquids have high hydrogen bonding
• this is because for a liquid to flow, the molecules must be able to flow past each other
• which means the constant breaking and reforming of intermolecular bonds
• so the stronger the bonds, the harder this becomes

Question 17

why are polar substances often soluble in water

Answer 17

this is because hydrogen bonds ca form between the water molecules and the substance that is being dissolved which helps the dissolving problem

Question 18

what is homolytic fission of haloalkanes and when does it occur

and write out the equation

Answer 18

• it is when the electrons shared between the carbon and the halogen each go to different atoms
• it occurs in the stratosphere when haloalkanes are exposed to intense ultraviolet radiation

CH3—Cl + hv —-> CH3* + Cl*

Question 19

when does heterolytic fission of haloalkanes occur and what does it form

Answer 19

• it is more common under laboratory conditions
• and since the carbon halogen bond is polar, it can break to form a negative halide ion and a positive carbocation

Question 20

define substitution

Answer 20

a reaction in which one atom or group in a molecule is replaces by another atom or group

Question 21

define nucleophile

Answer 21

a nucleophile is a molecule or negatively charged ion with a lone pair of electrons that it can donate to a positively charged atom to form a covalent bond

Question 22

describe the stages of nucleophilic substitution of a nucleophile on a haloalkane

Answer 22

-the nucleophile e.g. OH-, attacks the electron deficient carbon atom in the carbon-halogen bond
- the nucleophile donates two electrons to the carbon to form a dative covalent bond
- C–Br bond breaks heterolytically (they go the bromine atom)
- produces a negative bromine ion

Question 23

how does water act as a nucleophile