VERY SHORT ANSWER QUESTIONS (2 Marks Each)
Ques 1. State Markovnikov's rule.
Ans: According to Markovnikov's rule, the addition of unsymmetrical reagents on unsymmetrical alkenes takes place in such a way that the negative part of the addendum goes to the carbon carrying lesser number of hydrogen atoms.
CH3CH=CH2 + HBr ------->CH3CHBrCH3
Propene 2-Bromopropane
CH3 Cl
\ |
C=CH2 + HCI --------> CH3-------C-----CH3 CH3/ CH3 /
2-Methylpropene (iso-Butylene) 2-Chloro-2- methylpropane (tert-Buty! chloride).
Ques 2: Alkyl halides react with AgNO2 to give R-ONO2 and R-ONO. Why ?
Ans: Nitrite ion is an ambident nucleophile as it has two different nucleophilic sites, one is nitrogen atom and other is either of the two oxygen atoms. Reaction with unshared pair of N gives R-NO2 while P reaction with O gives R -O-N = O
R-X+Ag-0-N=O -------> R-O-N=O + RNO2
Ques 3: State anti Markovnikov's rule.
Ans: Anti Markovnikov's rules states that in the case of addition of HBr to alkene, the negative part of the attacking reagent (e.g. Br) will join to the carbon atom carrying more hydrogen atoms while H-atom will go to the other carbon atom containing lesser number of hydrogen atoms.
For example, CH3 - CH = CH2+ HBr------->CH3CH2 CH2Br Propene 1-Bromopropane
Ques 4: Why are haloarenes more stable than haloalkanes and undergo electrophilic substitution reactions at-ortho and para positions.
Ans: Haloarenes are stabilized due to resonance. A combined effect of (+R) resonance and
(-I) inductive effect in haloarenes gives rise to an increase in electron density at ortho- and para-
position reaction at these positions.
Ques 5: What is Wurtz reaction.
Ans: Wurtz reaction: When an ethereal solution of an aryl halide is heated with sodium metal and an alkyl halide, the halogen atom of aryl halide is replaced by the alkyl group. This reaction is called
Wurtz-Fittig reaction. For example,
ether
C6H6--Br +2 Na + CH3Br------> C6H6-----CH3 + 2NaBr
Small amounts of ethane and diphenyl are obtained, as side products.
Ques 6: What is dehydrohalogenation reaction?
Ans: When a haloalkanes containing beta-hydrogen atom(s) Is treated with an alcoholic.
hydroxide, one molecule of hydrogen halide (HCI,-
an alkene. This reaction is known as dehydrohalogenation. During this elimination process hydrogen atom is removed from the beta-carbon which is next to the carbon atom , as shown below
RCH2CH2Br--------------> RCH =CH2 + KBr + H2O
halolakane alkene
Ques 7: How will you prepare haloalkane from alcohol? Write one chemical reaction of each.
Ans: CH3CH2CH2OH + SOCl2 ----> CH3CH2CH2CI + SO2 + HCI
Ques 8: Alcohols are best converted into alkyl chlorides by reaction with thionyl chloride. Explain.
Ans: The reaction of alcohol with thionyl chloride results in formation of alkyl halide with simultaneous formation of HCl and SO2 which are gaseous in nature and hence, easily removed from reaction mixture, resulting in formation of almost pure alkyl chloride as shown below.
ROH + SOCl2 -----------> RCI +SO2 + HCI
Alcohol Thionyl Alkylchloride
Ques 9: Haloalkanes show nucleophilic substitution reaction. Explain.
Ans: Haloalkanes have polarity in C-X bond giving rise to partial negative charge on the halogen atom and partial positive charge on carbon atom. Due to the positive charge on the carbon carrying the halogen atom, a nucleophile attacks the carbon atom and replaces the halide ion to give a substituted product.
Ques 10: Haloarenes are insoluble in water but are soluble in benzene. Explain.
Ans: As haloarenes cannot form hydrogen bonding with water molecules, so haloarenes are not soluble in water. However, they are soluble in organic solvents because the intermolecular attractive forces in the benzene and the haloarenes are of similar nature, due to presence of a large hydrocarbon part in haloarenes and hence, they are soluble in benzene which itself is a hydrocarbon only.
Ques 11: What happens when;
(i) chloroform is exposed to air and sunlight.
(ii) carbon tetrachloride is hydrolysed by steam at high temperature ?
Air + light
Ans: (i) 2CHCl + O2 -----------> 2COCl2 +2HCI
Chloroform Phosgene
Cl OH
High temp. \ /
(ii)CCl4 + 2H2O -----------------> C (unstable)+
/ \ 2HCl
Cl OH
|
COCl2 + H2O
phosgene
Ques 12: Why the haloalkanes are only very slightly soluble in water ?
Ans: In order to dissolve haloalkane in water, energy is needed to overcome the attraction of haloalkane molecules and break the H-bonds between H,O molecules. Less energy is released
new attractions are set up between the haloalkane and the H2O molecules as these are not as the original H-bonds in water. As a result, the solubility of haloalkanes is less.
Ques 13: What are haloalkanes and haloarenes?
Ans: 1. Haloalkanes: These are derived from saturated hydrocarbons by replacing one or more hydrogen atoms by halogen atoms.
2. Haloarenes: These are derived from aromatic hydrocarbons by substituting one or more hydrogen atoms by corresponding number of halogen atoms.
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