3.3.3 - HALOGENOALKANES Flashcards
- what happens when an alkane is added to bromine water with UV light vs without
- why
- what does UV light do
Alkane + bromine water ➡️ stays red/brown
- Br cannot replace a H atom
Alkane + bromine water + UV ➡️ colourless + gas fumes
- UV provides energy to split Br2 into reactive radicals
Define the term “radical”
- a species with an unpaired electron
- highly reactive
What is radical substitution
- when a (diatomic) halogen is able to replace a H atom on a hydrocarbon
- other halogen atom forms HX (g) with the replaced H atom
What are the 3 stages of radical substitution
- Initiation (non radical + non radical ➡️ radical + radical)
- Propagation ( radical + non-radical ➡️ radical + non-radical)
- Termination (radical + radical ➡️ non-radical + non-radical)
Outline what happens in initiation
- UV energy absorbed by X - X bond
- halogen bond broken through homolytic fission
X — X ➡️ X* + X* - radicals very reactive
- outline what happens in propagation stage 1 and 2
PROPAGATION 1
- *X takes a H atom from alkane
- results in alkane free radical and HX
*X + CH4 ➡️ *CH3 + HX
PROPAGATION 2
- alkane radical reacts with a H2 molecule
- forms a stable halogenalkane and a halogen radical
*CH3 + X2 ➡️ CH3X + *X
- “chain” reaction
Outline what happens in the termination stage
- free radicals are removed
- correct radicals react to give the desired product
*X + *CH3 ➡️ CH3X
How could the further substituted halogenoalkanes (impurities) be decreased
Reduce the proportion of halogens in the reaction mixture
Explain a limitation of chain reactions (e.g. radical substitution)
- reactant may replace the “wrong” H atom, producing undesired isomers
- Ex. 1-chloropropane AND 2-chloropropane may be produced, which are difficult to separate
What is homolytic fission
When a bond breaks and 1 electron from each pair moves to each species, forming a radical
What is heterolytic fission
When a bond breaks and an electron pair moves to one species, producing 2 ions
- what is the ozone made out of
- what is it’s purpose
- O3 in atmosphere
- absorbs UV radiation
- protection from high energy UV rays
- state the equation for the formation of the ozone
- how does this occur
O2 (UV) ➡️ 2O*
- initiation: O—O bond broken, forming 2 O* radicals
O* + O2 ➡️ O3
- O* radical reacts with more O2 to form O3
State 2 ways ozone, O3, can be depleted
- (reverse formation) O3 ➡️ O* + O2 (occurs in equilibrium, no net change)
- CFC’s, where Cl catalyses depletion
- how do CFC’s (chlorofluorocarbons) catalyse the depletion of the ozone, O3?
- what are CFC’s used in
- used in refrigerators, aerosols
- gases at room temperature (since they are small, they have weak VdW’s)
- rise into the atmosphere, where C—Cl bonds are broken by UV light, where radicals are produced
State an equation for/ describe how CFC’s deplete the ozone, O3
CCl3F (UV) ➡️ CCl2F + Cl
- UV breaks the C—Cl bond
- Cl radical reacts with ozone to form radical, which reacts w more ozone to form 2 O2
State the reaction for depletion of the ozone, due to catalysts Cl*
Cl + O3 ➡️ ClO + O2
ClO* + O3 ➡️ 2 O2 + Cl*
OVERALL:
2 O3 ➡️ 3 O2
Which bond breaks in CFC’s (chlorofluorocarbons) in the presence of UV and why
- C—Cl
- it is a weak bond, requiring less energy
- what happens to the strength of C — X bonds, as you go down the halogen group?
- how does this influence reactivity
- strength decreases, as halogens get larger
- reactivity increases, as the bond breaks more easily
- what is an electrophile
- what does it do
- state an example
- species with + or δ+ charge
- attacks electron rich areas, “electron-liking”
- e.g. BF3, H+, H20
- what is a free radical
- what does it do
- species with an unpaired electron
- very reactive, as e- wants to “pair”
- attacks anywhere
- what is a nucleophile
- what does it do
- state an example
- species with negative or δ- charge
- attacks electron deficient areas, “nucleus (+) liking”
- e.g. OH-, NC-, NH3
State the 3 types of nucleophilic substitution and the nucleophiles involved
- OH-, forms alcohols
- NC-, forms nitriles
- NH3, forms amines
- state the conditions required for OH- nucleophilic substitution
- describe what is produced
- warm NaOH/KOH
- OH- replaces the X in a C — X bond
- forms C — OH, an alcohol
Outline the stages in nucleophilic substitution
- C — X bond is polar, due to electronegativity difference between the 2 elements
- Nucleophile is attracted to δ+ Carbon in the C — X bond
- Electrons in the C — X, are transferred to the X, due to it being highly electronegative, δ-
- Nucleophile replaces the halogen (X), forming a new product
- state conditions required for :NH3 substitution
- describe stages involved
- EXCESS, HOT, CONC. NH3
1. :NH3 attracted to δ+ C in C — X bond
2. Electrons in C — X move to the X, highly δ+
3. :NH3 replaces the halogen in the halogenoalkane
4. N becomes + charged, as it has used up its lone pair to bond, N — C
5. (Excess) :NH3 acts as a base, accepting the extra H (proton) on the NH3 branch
6. N becomes stable, forming an amine (NH2 group)
- what is formed in an elimination reaction
- how does an elimination reaction occur (conditions?)
- alkene + H2O + KX
- hot, ethanoic acid
- :OH-, nucleophile, acts as a base, “eliminates” a H atom
Outline the stages of an elimination reaction
- :OH- “eliminates” H atom adjacent to the halogen
- C—H bond breaks, electrons move to the C—C bond
- This forms a C = C double bond
- Second C now has 5 bonds, so must break one
- δ- X atom attracts electrons in the C—X bond, which now breaks
- Alkene forms
Outline the differences between nucleophilic substitution and elimination reactions, using :OH-
- conditions
- products
- stages
NUCLEOPHILIC SUBSTITUTION
- warm NaOH/KOH
- alcohols
- :OH- “substitutes” the X (halogen)
ELIMINATION
- hot, ethanoic KOH
- alkenes
- :OH- “eliminates” H atom, which “eliminates” X atom
