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XII Chem New Chap 06 Haloalkanes and Haloarenes 134 AR Items.
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Spread the Happiness. Assertion and Reason Statements Feedback: 9008271516XII_CHEMISTRY_NEW_CHAPTER-06: HALOALKANES AND HALOARENES _ A&R TEST ITEMS
# Correct Assertion Correct Reason
6.1 Classification; 6.1.1 On the Basis of Number of Halogen Atoms; 6.1.3 Compounds Containing sp2 C—X Bond
Haloalkanes and haloarenes are classified based on This classification system categorizes them as mono-, di-, or
1
the number of halogen atoms. polyhalogenated, reflecting the degree of halogen substitution.
Monohaloalkanes are further classified by the bonded This distinction considers the electronic environment (primary,
2 carbon's hybridization (sp3) attached to the halogen. secondary, or tertiary) around the carbon-halogen bond.
Allylic halides have the halogen bonded to a carbon This proximity to the electron-rich double bond can influence
3
near a double bond. reaction mechanisms, potentially increasing reactivity.
Benzylic halides have the halogen bonded to a carbon The aromatic ring's stability compared to typical sp3 carbons can
4
on an aromatic ring. influence reactivity, affecting reaction pathways.
Vinylic halides have the halogen bonded to an sp2 The delocalized electron system of the double bond can affect
5 carbon in a double bond. reactivity, leading to different behavior compared to sp3-halogen
bonds.
Aryl halides have the halogen directly bonded to an This placement integrates the halogen into the aromatic system's
6
aromatic ring's sp2 carbon. electronic structure, potentially affecting its properties.
Halogenated compounds persist in the environment The strong carbon-halogen bond and the electronegativity of
7 due to their resistance to breakdown. halogens hinder enzymatic degradation by soil bacteria, leading to
environmental persistence.
Haloalkanes and haloarenes are used as solvents for Their non-polar nature allows them to dissolve non-polar
8
non-polar compounds. compounds, making them useful in various industrial processes.
Haloalkanes and haloarenes serve as starting Their reactive halogen atom makes them versatile building blocks
9 materials for synthesizing various organic for creating a wide range of organic molecules.
compounds.
Haloalkanes contain halogen atoms bonded to sp3 Haloarenes, in contrast, have halogen atoms bonded to sp2
10
hybridized carbons. hybridized carbons of aromatic rings.
6.2 Nomenclature
Common names for alkyl halides prioritize naming the This naming convention prioritizes readability for simple
11 alkyl group followed by the halide (e.g., sec-butyl haloalkanes, directly identifying the substituent and the parent
chloride). chain.
IUPAC nomenclature for haloalkanes identifies them This systematic approach emphasizes the halogen as a functional
12 as halosubstituted hydrocarbons (e.g., 2- group attached to a hydrocarbon backbone, ensuring clear and
chlorobutane). unambiguous communication.
For disubstituted benzenes, common names use This naming system provides a quick reference for the relative
13 positional prefixes (o-, m-, p-) to denote halogen positions of two halogen substituents on a benzene ring, useful for
locations (e.g., o-dichlorobenzene). basic identification.
IUPAC nomenclature for dihalobenzenes utilizes This systematic approach offers a precise way to identify the specific
14 numerical designations (1,2; 1,3; 1,4) to indicate carbon atoms bearing the halogen substituents, critical for
halogen positions (e.g., 1,2-dichlorobenzene). differentiating isomers.
Geminal dihalides have both halogen atoms bonded The term "geminal" signifies that the two halogen substituents are
15
to the same carbon atom (e.g., 1,1-dichloroethane). twins (gemelli) directly attached to the same carbon.
Vicinal dihalides have halogen atoms bonded to The term "vicinal" indicates that the halogen substituents are
16
adjacent carbon atoms (e.g., 1,2-dichloroethane). neighbors (vicinus) on neighboring carbon atoms within the chain.
Common names for geminal dihalides use the prefix This naming convention identifies the presence of a geminal dihalo
17 "alkylidene" followed by the halide name (e.g., group, useful for simple cases where there's no ambiguity about
dichloromethane). substitution on the same carbon.
Common names for vicinal dihalides use the prefix This naming system emphasizes the presence of a vicinal dihalo
18 "alkylene" followed by "dihalide" (e.g., 1,2- group within an alkylene chain, informative for basic identification
dichloroethane). of vicinal isomers.
IUPAC nomenclature for both geminal and vicinal This systematic approach prioritizes identifying the core structure
19 dihalides uses "dihaloalkane" terminology (e.g., (dihaloalkane) without requiring positional differentiation for
dichloroethane). geminal/vicinal cases, ensuring clear classification.
Haloalkanes contain halogen atoms bonded to sp3- Haloarenes, in contrast, have halogen atoms bonded to sp2-
20 hybridized carbons, reflecting their tetrahedral hybridized carbons of aromatic rings, resulting in trigonal planar
geometry around the carbon-halogen bond. geometry due to delocalization of electrons within the ring.
Drawing structural isomers requires systematically This systematic approach ensures exploration of all possible
considering arrangements of carbon atoms and connectivity variations for a given set of atoms, leading to a
21
functional groups (halogens) while maintaining the complete identification of possible isomers.
same molecular formula (C5H11Br example).
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XII_CHEMISTRY_NEW_CHAPTER-06: HALOALKANES AND HALOARENES _ A&R TEST ITEMS
# Correct Assertion Correct Reason
IUPAC nomenclature prioritizes the longest carbon This systematic approach provides an unambiguous way to identify
chain for naming alkanes, and further incorporates the parent chain, the location of the halogen substituent, and any
22
prefixes (sec-, tert-) to denote branching near the branch points within the molecule.
halogen atom (e.g., 2-bromopentane).
6.3 Nature of C-X Bond
Electronegativity difference between carbon and The more electronegative halogen atom attracts the shared
halogen atoms leads to a polar C-X bond in alkyl electrons more towards itself, resulting in a partial negative charge
23
halides. and a partial positive charge on the halogen and carbon atoms
respectively
Increasing size of the halogen atom down the group Larger halogen atoms can accommodate a greater distance between
24 leads to a weaker carbon-halogen bond (C-F > C-Cl > C- their nucleus and the bonding electrons, leading to weaker
Br > C-I). electrostatic attraction and a decrease in bond strength.
The polarity of the C-X bond in alkyl halides A polar covalent bond creates an unequal distribution of charge,
25
contributes to their dipole moments. resulting in a measurable dipole moment for the molecule.
A trend of decreasing bond enthalpies (C-F > C-Cl > C- Weaker bonds require less energy to break, hence the bond
26 Br > C-I) along with increasing size of the halogen enthalpies decrease as the size of the halogen atom increases down
atom. the group, indicating a weaker bond strength.
Carbon-fluorine (C-F) bond exhibits the strongest The high electronegativity and small size of fluorine allow for
bond enthalpy and shortest bond length compared to greater overlap of orbitals and stronger electrostatic attraction in
27
other carbon-halogen bonds (C-X). the C-F bond, leading to a shorter bond length and higher bond
strength.
Weaker carbon-halogen bond strength in alkyl halides Weaker bonds are generally more prone to undergoing chemical
with larger halogen atoms might influence their reactions. This trend could be explored to predict or explain the
28
reactivity. relative reactivity of different alkyl halides depending on the
halogen atom present.
6.4 Methods of Preparation of Haloalkanes; 6.4.1 From Alcohols
Thionyl chloride (SOCl₂) is preferred for alkyl halide SOCl₂ forms removable gases (SO₂ & HCl), preventing water
29
preparation. (hydrolysis) and promoting a cleaner reaction.
ZnCl₂ is required as a catalyst for HCl reaction with Low HCl dissociation limits free Cl⁻ ions. ZnCl₂ acts as a Lewis acid,
30
primary/secondary alcohols. forming a more reactive electrophilic species (ZnCl⁺).
Tertiary alcohols react faster with haloacids than Increased electron donation from alkyl groups enhances
31
primary or secondary alcohols. nucleophilicity of the oxygen atom in tertiary alcohols.
Selection of halogenating agent dictates the final Different agents provide varying halide ions (Cl⁻, Br⁻, I⁻) that act as
32
product's halogen atom. nucleophiles, determining the incorporated halogen.
Heating is necessary for preparing alkyl bromides Thermal energy increases reactant molecule collisions, overcoming
33
using HBr. the activation energy barrier for the reaction.
The stronger C-O bond in phenols hinders conversion The partial double bond character strengthens the C-O bond in
34
to aryl halides using simple haloacids. phenols, making it less susceptible to nucleophilic attack.
In situ generation of phosphorus trihalides simplifies Commercially unavailable phosphorus trihalides (PBr₃ & PI₃) can be
35 alkyl bromide and iodide preparation. directly generated in the reaction mixture (in situ), simplifying the
process.
6.4.2 From Hydrocarbons
Free radical halogenation is a substitution reaction. It replaces a hydrogen atom in an alkane with a halogen (Cl, Br, I)
36
via a free radical mechanism.
Multiple reaction pathways reduce the yield of any Competing pathways for hydrogen abstraction from the alkane
37 single isomer. molecule limit the formation of a specific isomer by consuming
starting material.
Markovnikov's rule is not applicable to free radical Markovnikov's rule predicts regioselectivity in alkene addition
38 halogenation. reactions based on carbocation stability, while free radical
halogenation involves random hydrogen abstraction.
Equivalent C-H bonds in alkanes allow for various Each C-H bond offers an equally likely site for chlorine radical attack,
39 halogenated products. leading to a diversity of isomers depending on the abstraction
location.
6.4.3 Halogen Exchange
Finkelstein reaction uses NaI in dry acetone for alkyl Precipitated NaCl/NaBr removal in dry acetone shifts equilibrium
40
iodides. towards product formation (Le Chatelier's principle).
Dry acetone is essential for the Finkelstein reaction. Non-polar dry acetone facilitates precipitation of ionic NaCl/NaBr,
41
removing them from solution and driving the equilibrium forward.
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