organic chemistry haloalkanes

Question 1

what is a halogenalkane

Answer 1

a compound which has an R–X R (alkyl group eg. alkane or alkene chain) X (any halogen atom)

Question 2

what is the general formula for a halogenalkane

Answer 2

monosubstituted : C_nH_2n+1+X

Question 3

explain why there is bond polarity of halogenalkanes

Answer 3

have a C—X bond

This is a polar bond with the Carbon being delta positive (electron deficient) and the halogen being delta negative

As go down group of halogens they get less polar as difference in electronegativety decreases

Question 4

explain the different types of halogenalkanes

Answer 4

can be classified as primary, secondary or tertiary

depends upon the number of alkyl groups attached to the carbon which is bonded with the halogen

Question 5

explain the trend in boiling points with halogenalkanes

Answer 5

the boiling point of simple molecular substances depend on the strength of the intermoleular forces

Halogenalkanes form dipole-dipole and van der waals

strength depends on

: increase BP with increase in carbon chain length

: increase BP as size of halogen increases

: decrease in BP with increase in branching

have higher boiling points than alkanes as they have higher relative masses and are more polar with permanent dipole dipole forces

Question 6

explain the solubility of halogenalkanes

Answer 6

the C—-X bonds are not polar enough to make them souluble in water as the majority of the molecule is not polar

tend to be more soluble in non-polar solvents eg. hexane

Question 7

what are the two factors that determine how halogenalkanes react

Answer 7

almost always the C—X bond that breaks

two main factors that determine how easily this bond breaks

1) bond polarity
2) bond enthalpy

Question 8

explain the polar bonds with haloalkanes with susceptibility to an attack from a nucleophile

Answer 8

The high electronegativity of the halogen atom causes the carbon to become electron deficient and susceptible to an attack from a nucleophile

Nucleophile is a species that is a electron pair donor

This theory would predict C-F bond would be the most reactive as most polar and susceptible to an attack from a nucleophile

Question 9

explain how bond enthalpy explains the reactivity of the haloalkanes

Answer 9

The bond enthalpy decreases as you go down the group

this is because the halogen atom is larger meaning the C–X bond must be longer making it weaker

This predicts that iodo compounds must be the most reactive as they have the longest and weakest bonds and floro compounds are the least reactive

experiments confirm that reactivity increases going down the group, this means that bond enthalpy is a more important factor than bond polarity

Question 10

what is nucleophilic substitution

Answer 10

Haloalkanes are susceptible to attack of nucleophiles (electron pair donors) such as OH^- CN^- and NH₃

in a substitution reaction the halogen atom is replaced by another atom/group

Question 11

show an example of NS with warm aqueous NaOH

Answer 11

reagents : warm aqueous NaOH (+haloalkane)

conditions : warm aqueous

Nucleophile : hydroxide ion :OH^-

what happens : halogen atom is replaced by OH

overall equation : R—–X + NaOH → R—–OH + NaX

mechanism :

Question 12

show the equation and mechanism for 2-iodo-3-methylbutane with aqueous NaOH

Answer 12

Question 13

show an example of NS with KCN

Answer 13

reagents : warm aqueous ethanoic potassium cyanide (KCN) (+haloalkane)

conditions : warm aqueous ethanoic

Nucleophile : cyanide ion :CN^-

what happens : halogen atom is replaced by CN

overall equation : R—–X + KCN → R—–CN + KX

mechanism :

Question 14

show the equation and mechanism for 1-bromobutane and ethanolic KCN

Answer 14

Question 15

show an example of NS with NH₃

Answer 15

reagents : ammonia (NH₃) (+haloalkane)

conditions : excess concentrated NH₃ dissolved in ethanol, under pressure in a sealed container

Nucleophile : ammonia :NH₃ , 2 molecules

what happens : First M: halogen atom is replaced by a NH₂

second M: leads to the formation to NH₄X

Question 16

show the equation and mechanism of 2-bromo-3-methylbutane in an excess of conc NH₃

Answer 16

Question 17

explain elimination mechanisms with haloalkanes

Answer 17

if reaction conditions are changed they can react to form alkenes

In this elimination reaction the hydroxide ion acts as a base to remove a proton (H⁺) from the alkane , the electron in the C-H bond double between the C that has lost the proton and the C atom which is attached to the halogen

Question 18

show and explain the mechanism and equation of NaOH and KOH in ethanol

Answer 18

reagents : NaOH/KOH

conditions : ethanolic conditions + hot reflux

what happens : halogen atom and one hydrogen atom from the adjacent C atom are removed giving an alkene

• elimination can not happen if there is no hydrogen on a adjacent C

Question 19

show and explain the mechanism of 2-chloropropane and hot ethanolic KOH

Answer 19

Question 20

show the mechanism of 2-bromobutane + hot ethanolic KOH to give but-2-ene

Answer 20

Question 21

explain how the conditions can determine what kind of haloalkane is made

Answer 21

Question 22

explain how elimination mechanisms can give different products depending if the halo alkane is primary, seconday or tertiary

Answer 22

Question 23

what is the ozone layer

Answer 23

The ozone layer is found in the stratosphere, ozone molecules O₃ are present in high concentrations and continually formed and broken down by intense short wavelength UV radiation. They reduce amount of harmful radiation that reaches earth responsible for sun burn, skin cancer etc

Question 24

show and explain the two step reaction to form ozone

Answer 24

1) oxygen molecules O₂ are exposed to UV light causing homolytic bond breaking to happen forming 2 oxygen free radicals (species with unpaired e)

O₂ → 2O^•

2) an oxygen free radical reacts with an oxygen molecule (O₂) to form ozone (O₃)

O^• + O₂ → O₃

Question 25

explain the break down of ozone

Answer 25

the breakdown occurs on the absorption of UV light (at the wavelength that damages biological life). This forms an oxygen molecule (O₂) and a oxygen free radical

O₃ → O₂ + O^•

without ozone depleting pollutants in the atmosphere the concentration of ozone would remain at constant as it is formed at the equal rate as its broken down

Question 26

explain and describe what CFCs are

Answer 26

CFCs are chloroflorocarbons, they all have the hydrogen atoms replaced by either a chlorine or a fluorine. They are used for anasthetics, refrigerants and aerosol propellants as they are unreactive under standard conditions and are volatile

examples include trichlorofluoromethane and 1,1,2-trichloro-1,2,2- trifluoroethane

Question 27

explain and describe how CFCs have aa detrimental effect on the ozone layer

Answer 27

CFCs are unreactive as the C-F and C-Cl bonds are relatively strong, therefore they can exist in the atmosphere

when these molecules reach the stratosphere the high energy UV light from the sun provides enough energy to break the C-Cl bond homolytically, prefer over C-F as C-Cl bond is weaker

Question 28

show and explain the overall equation for the decomposition of ozone

Answer 28

The chlorine free radical is regenerated (acts as a catalyst) and goes on to react further. Allows the breakdown of ozone via an alternate reaction pathway of lower activation energy.

Question 29

explain what we use for replacements of CFCs

Answer 29

hydrofluorocarbons (HFC) are noew used

Only contain fluorine not Cl

C-F bond is much stronger than the C-Cl so are less reactive, not broken down in the stratosphere as the UV light does not provide sufficient energy and a free radical is not produced