Halogen Derivatives Flashcards
Why do halogenoalkanes and halogenoarenes have poor solubility in water?
They form permanent dipole-permanent dipole interactions with water molecules. The energy that is released from this interaction is insufficient to overcome the strong hydrogen bonding between water molecules.
Why are halogenoalkanes and halogenoarenes soluble in non-polar solvents?
This is due to the presence of non-polar alkyl groups or benzene rings of the respective compounds and thus can form instantaneous dipole-induced dipole attractions of similar
strength with the non-polar solvent molecules.
Why are halogenoalkanes and halogenoarenes dense?
Halogenoalkanes and halogenoarenes are generally denser than both water and the alkane or arene which they are derived from. This is due to the presence of the halogen atom, which increases the molar mass.
State the reagents and conditions to prepare halogenoalkanes from alkanes.
Free Radical Substitution of alkanes.
Reagents and conditions: Cl2(g) or Br2(l); uv light
Observations: *Greenish-yellow Cl2(g) or reddish-brown Br2(l) decolourises
Free radical substitution of alkanes is seldom used due to the formation of a mixture of products
State the reagents and conditions to prepare halogenoalkanes from alkenes.
Electrophilic Addition of hydrogen hallides.
Reagents and conditions: HCl(g), HBr(g) or HI(g); room temperature
OR
Electrophilic Addition of halogen.
Reagents and conditions: Cl2 in CCl4 or Br2 in CCl4; room temperature, in the dark
Observation: *Greenish-yellow Cl2 or orange-red Br2 decolourises
The use of Cl2 for distinguishing tests is thus not advisable. The decolourisation of greenish yellow Cl2 gas or pale yellow Cl2 in CCl4 may be difficult to observe depending on the intensity of the colour.
State the reagents and conditions to prepare halogenoalkanes from alcohols.
Nucleophilic Substitution of Alcohols.
To form R-X:
1. HCl(g)/HBr(g)/HI(g), r.t
Forms: R-X and H2O
2. anhydrous PCl3/PBr3/PI3, heat
Forms: R-X + H3PO4
To form R-Cl:
3. anhydrous PCl5, r.t
Forms: R-Cl + POCl3 + HCl
4. anhydrous SOCl2, heat
Forms’: R-Cl + SO2 + HCl
Observations for (3) and (4): white fumes of HCl are produced, which turn damp blue litmus paper red
The advantage of using SOCl2 is that the two by-products are both gases (SO2 and HCl), so they separate themselves from the RCl product easily.
State the reagents and conditions to prepare halogenoarenes from arenes.
Reagents and conditions: X2 with anhydrous FeX3 or anhydrous AlX3 or Fe as catalyst; room temperature [X = Cl or Br]
Why can halgenoalkanes undergo nucleophilic substitution?
- Halogenoalkanes are often polar molecules with the C atom of the C–X bond having a partial positive charge. This makes this C atom electron-deficient.
- Nucleophilic substitution reactions occur when a nucleophile (electron pair donor, Lewis Base) is attracted to the electron-deficient C atom (Lewis Acid), which is susceptible to attack by these nucleophiles, where the nucleophile substitutes the halogen atom.
What is a SN2 reaction?
It is a mechanism where two species are involved in the rate determing step.
The rate equation of a SN2 mechanism is rate = k[RX][nucleophile], where the rate is dependent on both RX and the nucleophile
SN2 (subsitution, nucleophile, bimolecular)
What happens if the halogenoalkane has a chiral a-carbon during a SN2 reaction?
If the halogenoalkane has a chiral a-carbon, due to the rear-side attack, there will be an inversion of configuration at the a-carbon.
Why do primary halogenoalkanes undergo SN2 reactions more readily?
Steric hindrance increases as additional alkyl groups are bonded to the a–carbon. Hence, generally primary halogenoalkanes undergo SN2 reactions more readily and tertiary halogenoalkanes do not.
The rate of the nucleophilic substitution reaction
decreases from methyl > 1° > 2° > 3° halogenoalkane.
What is a SN1 reaction?
It is a reaction where only one species is involved in the rate-determining step.The derived rate equation for this mechanism is rate = k[RX], where the rate is dependent on the concentration of RX only.
SN1 (Substitution, Nucleophilic, unimolecular)
Why can a racemic mixture be formed during a SN1 reaction?
The carbocation intermediate is trigonal planar with respect to the cationic carbon, hence, the nucleophile can attack from either face (top and bottom) with equal probability.
If nucleophilic attack of the electron-deficient carbon gives rise to a chiral carbon, a racemic mixture is formed as both enantiomers would be formed in equimolar amounts.
What affects the stability of the carbocation in a SN1 reaction?
Stability of carbocation increases as additional electron-donating alkyl groups are bonded to the positively charged carbon. Hence, generally tertiary halogenoalkanes undergo SN1 reactions more readily and primary halogenoalkanes do not and the rate of the nucleophilic substitution reaction decreases from 3° > 2° > 1° > methyl
halogenoalkane.
What affects the reactivity of halogenoalkanes in nucleophilic substiution and how does it vary down the galogen group?
The reactivity of halogenoalkanes by nucleophilic substitution reactions depends on the ease of cleavage of the C–X bond. The stronger the bond, the harder it is for the bond to break.
As the size of the halogens become larger from F to I, the valence orbital used for bonding becomes larger and more diffuse. Thus, the orbital overlap between the halogen and carbon atoms becomes less effective and bond strength decreases down the group.
Due to the very strong C–F bond, fluoroalkanes are generally unreactive and do not undergo nucleophilic substitution.
State the reagents and conditions to form alcohols from halogenoalkanes and any observations.
Reagents and conditions: NaOH(aq) or KOH(aq); heat
This reaction is also known as alkaline hydrolysis,
State the reagents and conditions to form nitriles from halogenoalkanes and any observations.
Reagents and conditions: KCN or NaCN, ethanol; heat
This is a step up formation as the carbon chain is extended by one carbon
State the reagents and conditions to form amides from halogenoalkanes and any observations.
Reagents and conditions: excess NH3, ethanol; heat in sealed tube
A primary amine is formed when a halogenoalkanes reacts with ammonia. Excess ammonia is used to ensure high yield of the primary amine.
State the reagents and conditions to react halogenoalkanes to alkenes.
Reagents and conditions; NaOH or KOH, ethanol; heat
Why don’t halogenoarenes undergo nucleophilic substitution?
- The C-X bind is strengthened and not easily cleaved. The p orbital of the halogen atom can overlap sideway with the pi electron cloud of the benzene ring, resulting in the lone pair of electrons on the halogen atom delocalising into the benzene ring. This leads to a partial double bond character in the C-X bond which makes it stronger than the C-X bond in halogenoalkanes.
- Steric hindrance. The benzene ring sterically hinders SN2 reactions. The C in the C-C bond is not accessible to nucleophilic attack via rear side attack.
Why do halogenoarenes undergo electrophilic substitution at a lower rate than benzene rings?
Electrophilic substitution of halogenoalkenes happen at a lower rate because the halogen atoms are very electronegative, thus decreasing electron density of the pi electron cloud of the benzene ring.
State the steps of the distinguishing test for halogen derivatives.
Step 1: NaOH(aq) snd heat
Step 2: Excess dilute HNO3 after cooling
Step 3: AgNO3(aq) and observe for any precipitation that may occur.
How do you distinguish the halogenoalkanes and halogenoarenes with the distinguishing test?
A precipitate would not be formed when halogenoalkanes and flouroalkands undergo the chemical test due to the strong C-C bond.
R-Cl: white ppt of AgCl(s) formed
R-Br: cream ppt AgBr(s) formed
R-I: yellow ppt of AgI(s) formed
