halogen derivatives

Question 1

what are halogenoalkanes?

Answer 1

halogenoalkanes are alkanes with one or more H atoms replaced by the halogen atom, F, Cl, Br or I

primary halogenoalkanes: halogenated carbon is only attached to 1 alkyl group
secondary halogenoalkanes: halogenated carbon is attached to 2 alkyl groups
tertiary halogenoalkanes: halogenated carbon is attached to 3 alkyl groups

Question 2

what are some physical properties of halogenoalkanes?

Answer 2

1. boiling point of halogenoalkanes are higher than their corresponding alkanes (w same no. of C)
2. for the same alkyl group, boiling point increases down the group (chloroalkane < bromoalkanes < iodoalkane)
3. halogenoalkanes are generally colourless liquids with sweetiish smells
4. halogenoalkanes are immiscible with and denser than water

Question 3

why does boiling point increase in the order chloroethane < bromoethane < iodoethane?

Answer 3

• all 3 compounds are simple covalent molecules
• number of electrons, hence size of electron cloud, increases in the order chloroethane < bromoethane < iodoethane
• hence, intermolecular dispersion forces increases in strength in the same order
• energy needed to overcome intermolecular attractions increases in the same order, hence boiling point increases in the same order

Question 4

why does chloropropane has a higher boiling point than propane?

Answer 4

• both chloropropane & propane are simple covalent molecules
• chloropropane has more electrons than propane, hence a larger electron cloud, so chloropropane forms stronger dispersion forces between its molecules
• in addition, chloropropane has some amount of permanent dipole-permanent dipole attraction
• thus, more energy is required to overcome stronger intermolecular attractions between chloropropane molecules during boiling than between propane molecules. hence, chloropropane has a higher boiling point

Question 5

what are halogenoarenes?

Answer 5

halogenoarenes have a halogen atom attached directly to a benzene ring

Question 6

how can halogen derivatives be prepared?

Answer 6

halogenoalkanes
1. free radical substitution of alkanes (but would get a mixture of halogenated alkanes)
2. electrophilic addiion on alkenes (might get 2 products if alkene is asymmetrical)
3. nucleophilic substitution of alcohols

halogenoarenes
1. electrophilic substitution of arenes

Question 7

why do halogenoalkanes undergo nucleophilic substitution?

Answer 7

• halogens are more electronegative than carbon, so the carbon-halogen bond is polar
• the electron-deficient carbon atom has a partial positive charge (electron deficient) which causes it to be attractive to nucleophiles (electron rich)
• during reaction, the halogen atom can be substituted by a nucleophile

Question 8

which halogenoalkanes undergo Sn1 reaction?

Answer 8

secondary & tertiary halogenoalkanes undergo Sn1 reaction.

primary & secondary halogenoalkanes undergo Sn2 reaction

Question 9

is it possible for an optically active product to be obtained through the Sn2 mechanism?

Answer 9

yes!

if the original halogenoalkane molecule is chiral, and the reaction occurs purely via the Sn2 mechanism (i.e. primary halogenoalkane), only 1 of the enantiomers will be formed as the product. there will not be a mixture of the 2 enantiomers, so it will be optically active

Question 10

how can a racemic mixture be formed through the Sn1 mechanism?

Answer 10

• the carbocation intermediate is trigonal planar with respect to the electron-deficient carbon
• so, the nucleophile is able to attack from the top and bottom face of the carbocation with equal probability
• if the addition of the nucleophile results in a chiral product (4 diff groups attached to 1 carbon), a racemic mixture is formed as both mirror image enantiomers are formed in equal quantities

Question 11

LO: why do primary halogenoalkanes undergo Sn2 and not Sn1?

Answer 11

• for primary halogenoalkanes, the nucleophile can easily approach the electron deficient carbon as there is little steric hindrance as primary halogenoalkanes have only one alkyl group
• primary halogenoalkanes cannot undergo Sn1 as the primary carbocation intermediate formed is relatively unstable as there is only one electron-donating alkyl group attached to the positively-charged carbon atom

Question 12

LO: why do tertiary halogenoalkanes undergo Sn1 and not Sn2?

Answer 12

• for tertiary halogenoalkanes, the tertiary carbocation formed is relatively stable as there are 3 electron-donating alkyl groups attached to the positively charged carbon atom
• tertiary halogenoalkanes cannot undergo Sn2 because of the steric hindrance from the 3 alkyl groups that obstruct the approach of the nucleophile towards the electron deficient carbon

Question 13

LO: why does reactivity of halogenoalkanes increase in the order: fluoroalkane < chloroalkane < bromoalkane < iodoalkane?

Answer 13

• as the size of the halogen atom increases from F < Cl < Br < I, the C-X bond length increases, and the bond strength decreases, so nucleophilic substitution takes place more readily in the order fluoroalkane < chloroalkane < bromoalkane < iodoalkane

Question 14

LO: why are halogenoarenes much less susceptible to nucleophilic substitution than halogenoalkanes?

Answer 14

• the lone pair of electrons on the halogen atom delocalises into the benzene ring
• this results in partial double bond character in the C-X bond, so its bond length is shorter and hence its bond strength is stronger than its corresponding halogenoalkane -> the C-X bond in halogenoarenes is thus very difficult to break
• sterically, the bulky benzene ring hinders/blocks the attack of a nucleophile on the halogenated carbon
• the pi electron cloud of the benzene ring repels the lone pair of electrons of an incoming nucleophile, making nucleophilic attack difficult

so nucleophilic sub of halogenoarenes can only be achieved under very vigorous conditions

Question 15

can halogenoarenes undergo electrophilic substitution?

Answer 15

yes! (refer to arenes notes)

but halogenoarenes are less susceptible to electropholic sub as the halogen substituent is deactivating (and 2,4-directing)

Question 16

LO: how to distinguish between halogenoalkanes by comparing the colour and solubility of AgX precipitate?

Answer 16

• add NaOH (aq) and heat
• add excess dilute HNO3
• add AgNO3 (aq) and observe the colour of ppt formed
• (to differentiate between chloroalkane & bromoalkane) decant & wash ppt w deionised water, and add NH3 (aq) to check for solubility

results

• chloroalkane: white ppt of AgCl formed, dissolves in dilute NH3 (aq)
• bromoalkane: cream ppt of AgBr formed, dissolves in conc NH3 (aq)
• iodoalkane: yellow ppt of AgI formed, insoluble even in conc NH3 (aq)

halogenoarenes will NOT react with aqueous silver nitrate

Question 17

LO: how to distinguish between halogenoalkanes by comparing the rate of formation of silver halide ppt?

Answer 17

• add 1cm3 of the organic compound into a test tube
• add 1cm3 of ethanolic AgNO3 and place the test tube into a hot water bath

results

• chloroalkane: white ppt of AgCl is formed in a longer time compared to bromoalkane
• bromoalkane: cream ppt of AgBr is formed in a longer time compared to iodoalkane
• iodoalkane: yellow ppt of AgI formed almost immediately

Question 18

what are chlorofluorocarbons (CFCs) used for?

Answer 18

1. refrigerants
2. aerosol propellants
3. fire extinguishers

Question 19

LO: why is depletion of ozone layer concerning?

Answer 19

the ozone layer absorbs UV rays and screens the earth from excess UV radiation. too much exposure to UV radiation causes skin cancer, damages eyes and can lead to widespread crop death

Question 20

what are measures that can protect the ozone layer?

Answer 20

• reduce use of CFCs in industrial & household applications
• use substitutes for CFCs like hydrocarbons, hydrofluorocarbons or fluorocarbons.

Question 21

reagents & conditions for nucleophilic sub of halogenoalkanes with OH- to form alcohols?

Answer 21

dilute NaOH, heat

Question 22

reagents & conditions for nucleophilic sub of halogenoalkanes with CN- to form nitriles?

Answer 22

ethanolic KCN, heat

step up reaction

Question 23

reagents & conditions for acidic hydrolysis of nitriles to form carboxylic acids?

Answer 23

dilute H2SO4, heat

Question 24

reagents & conditions for alkaline hydrolysis of nitriles to form carboxylates?

Answer 24

dilute NaOH, heat

Question 25

reagents & conditions for reduction of nitriles to form amines?

Answer 25

LiAlH4 in dry ether OR H2 (g), Ni catalyst, high pressure

Question 25

reagents & conditions for nucleophilic sub of halogenoalkanes with NH3 to form amines?

Answer 25

ethanolic conc NH3, heat in sealed tube

Question 26

reagents & conditions for elimination fo halogenoalkanes to form alkenes?

Answer 26

ethanolic KOH, heat

Question 27

LO: a test to differentiate between halogenoalkanes & halogenoarenes?

Answer 27

heat with AgNO3 halogenoalkanes will react (ppt formed, colour depends on what halogen) halogenoarenes will not react (no ppt/no observable change)

Question 28

LO: what characteristic of fluoroalkanes & fluorohalogenoalkanes makes them useful for non-stick pan coating & stoppers in laboratory glassware?

Answer 28

fluoroalkanes & fluorohalogenoalkanes are relatively inert due to the **high bond energy of the C-F bond**, so they are less reactive than chloroalkanes.