Electron Sharing Reactions

விளையாடிக் கற்றுக்கொள்ளுங்கள்

ஆற்றல் சம்பாதிக்க இந்த கேள்விகளுக்குப் பதிலளியுங்கள், பின்னர் மீன் பிடித்து ஆராயுங்கள். கணக்கு தேவையில்லை.

கல்வியாளர்களுக்கு: Electron Sharing Reactions (Chemistry, SL)-க்கான தயாரான பாட ஸ்லைடுகள், திருப்புதல் குறிப்புகள் — உங்கள் பாடத்தில் அவற்றைப் பயன்படுத்தவும், அல்லது கற்பவர்கள் நேரலை விளையாட்டாக விளையாடும் ஊடாடும் வகுப்பு செயல்பாடாக தலைப்பை இயக்கவும்.

பாட குறிப்புகள்

What Are Radicals?

A radical is a chemical species that has an unpaired electron.
Radicals can be atomic (a single atom with an unpaired electron), polyatomic/molecular (a group of bonded atoms with no overall charge and an unpaired electron), anionic (a negatively charged species with an unpaired electron), or cationic (a positively charged species with an unpaired electron).
The sole requirement for a radical is the unpaired electron; radicals can exist independent of the charge on the species.
Radicals are indicated by a dot (•) in the formula, placed on the atom with the unpaired electron.
Examples include Cl•, Br•, CH3•, OH•, NO•, and •C6H6– (anionic).

Reactivity of Radicals

Radicals are highly reactive due to their unpaired electron, which gives them high enthalpy.
To lower their enthalpy, radicals react quickly by stealing an electron from another species (which then becomes a new radical) or by combining with another radical to form a covalent bond.
Because of this high reactivity, radicals are usually short-lived.

Homolytic Fission

Homolytic fission is the breaking of a covalent bond such that each atom takes one electron from the bond, forming two radicals.
In mechanisms, single-headed curly arrows (fish-hook arrows) show the movement of a single electron.
Curly arrows must start at an electron-rich region (e.g., the middle of the covalent bond) and finish at the correct destination (e.g., each chlorine atom).
Bond breaking is endothermic, so energy is required; the amount depends on the strength of the covalent bond.
Thermolytic fission: weaker bonds can be broken by heating (e.g., X:X → 2X• with heat).
Photolytic fission: stronger bonds, such as halogen bonds, require high-energy UV light (e.g., X:X → 2X• with UV light).

Stability of Alkanes

Alkanes are relatively unreactive due to the strengths of C–C and C–H bonds and their non-polar nature.
The C–C and C–H covalent bonds are strong, so a lot of heat is needed to break them, decreasing reactivity.
Carbon and hydrogen have almost the same electronegativity (difference of only 0.4 on the Pauling scale), so electrons are shared almost equally.
As a result, alkanes are nonpolar and have no partial positive or negative charges (δ+ and δ–).
Alkanes do not react with polar reagents; they have no electron-deficient areas to attract nucleophiles and no electron-rich areas to attract electrophiles.
Alkanes only react in combustion reactions and undergo substitution by radicals.

Free-Radical Substitution of Alkanes

Alkanes undergo free-radical substitution in which a hydrogen atom is substituted by a halogen (chlorine or bromine).
Ultraviolet light (sunlight) is needed for this substitution reaction to occur.
The reaction consists of three steps: initiation, propagation, and termination.
This reaction is not suitable for preparing specific halogenoalkanes because a mixture of substitution products is formed.
If enough halogen is present, all hydrogens in the alkane can eventually be substituted (e.g., up to hexachloroethane, C2Cl6).

Initiation Step

In the initiation step, the halogen bond (Cl–Cl or Br–Br) is broken by UV energy to form two radicals.
The covalent bond breaks by homolytic fission, with each atom taking one electron.
Example: Cl–Cl → 2Cl• (with UV light).

Propagation Steps

Halogen free radicals are very reactive and attack the unreactive alkanes.
One C–H bond in the alkane breaks homolytically to produce an alkyl radical and hydrogen halide: CH4 + Cl• → •CH3 + HCl.
The alkyl radical attacks another halogen molecule to form a halogenoalkane and regenerate the halogen radical: •CH3 + Cl2 → CH3Cl + Cl•.
The regenerated halogen radical repeats the cycle.
For ethane: CH3CH3 + Cl• → CH3CH2• + HCl and CH3CH2• + Cl2 → CH3CH2Cl + Cl•.
Further substitution can occur: CH3CH2Cl + Cl• → •CH2CH2Cl + HCl and •CH2CH2Cl + Cl2 → CH2ClCH2Cl + Cl• (1,2-dichloroethane).
Examiner tip: Do not show formation of a hydrogen radical in propagation; e.g., CH3CH2• + Cl2 → CH3CH2Cl + H• does not happen.

Termination Steps

Termination occurs when two free radicals react together to form a single unreactive molecule, stopping the chain reaction.
Multiple products are possible.
Examples for ethane and chlorine: CH3CH2• + Cl• → CH3CH2Cl (chloroethane).
CH3CH2• + •CH2CH3 → CH3CH2CH2CH3 (butane).
Cl• + Cl• → Cl2 (chlorine molecule).

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பயிற்சி கேள்விகள்

இலவச முன்னோட்டம் — 63-இல் 8 கேள்விகள். அனைத்தையும் பார்க்க பதிவு செய்யவும்.

1.Which of these organic compounds would undergo free radical substitution? I. ethane II. fluoroethane III. ethene
Easy
- AI only
- BIII only
- CI and II only
- DI, II and III
2.Which equation represents a correct propagation step in the free radical substitution reaction between ethane and chlorine?
Easy
- AC2H6 + Cl• → C2H5Cl + H•
- BC2H5• + Cl2 → C2H5Cl + Cl•
- CC2H6 + H• → C2H5• + HCl
- DC2H5• + Cl• → C2H5Cl
3.Ultraviolet light initiates the following reaction. alkane + chlorine → chloroalkane + hydrogen chloride What happens to chlorine in this photochemical reaction?
Easy
- Aheterolytic fission to give an electrophile
- Bhomolytic fission to give an electrophile
- Cheterolytic fission to give a positive and negative ion
- Dhomolytic fission to give free radicals
4.In the presence of ultraviolet light, ethane and chlorine react to give a mixture of products. Which compound could be present in the mixture of products?
Medium
- ACH3Cl
- BCH3CH2CH2Cl
- CCH3CH2CH2CH3
- DCH3CH2CH2CH2CH3
5.Which statements about the chlorine free radical are correct? I. It has 17 electrons. II. It is an uncharged species. III. It is formed by homolytic fission.
Medium
- AI and II only
- BI and III only
- CII and III only
- DI, II and III
6.Which organic product does not form in the following reaction? C2H6 + Cl2 → (UV)
Medium
- A1,2-dichloroethane (CH2Cl–CH2Cl)
- BChloroethane (C2H5Cl)
- CChloromethane (CH3Cl)
- D1,1-dichloroethane (CH3–CHCl2)
7.Ethane reacts with bromine in the presence of ultraviolet light to form dibromoethane and hydrogen bromide. The reaction proceeds via free radical substitution involving initiation, propagation and termination steps. What is the minimum number of free radical mechanism steps to produce dibromoethane?
Hard
- A5
- B6
- C7
- D8
8.Ethane reacts with chlorine in the presence of ultraviolet light to form trichloroethane and hydrogen chloride. The reaction proceeds via free radical substitution involving initiation, propagation and termination steps. Which of the following is a valid propagation step in the mechanism for the formation of trichloroethane?
Hard
- AC2H6 + Cl• → C2H6• + HCl
- BC2H3Cl2 + Cl• → C2H3Cl3 + H•
- CC2H5• + Cl2 → C2H5Cl + Cl•
- DC2H4Cl• + Cl2 → C2H4Cl2 + Cl•

மேலும் கேள்விகள்

9.Ethane reacts with bromine in the presence of ultraviolet light to form tribromoethane and hydrogen bromide. The reaction proceeds via free radical substitution involving initiation, propagation and termination steps. Which equation represents a possible termination step in this reaction?
Hard
- A2C2H4• → C4H8
- BC2H4Br• + C2H5• → C4H9Br
- CC2H4• + C2H3Br• → C4H7Br
- DC2H3Br2• + C2H3Br2• → C4H6Br4
10.Which of the following best describes homolytic fission?
Medium
- ABreaking a covalent bond so that one atom takes both electrons, forming ions
- BBreaking a covalent bond so that each atom takes one electron, forming radicals
- CBreaking a covalent bond using UV light to form ions
- DBreaking a covalent bond using heat to form ions
11.Which of the following statements about radicals are correct? (select all that apply)
Medium
- AA radical is a chemical species with an unpaired electron.
- BAll radicals are positively charged.
- CRadicals are highly reactive due to their unpaired electron.
- DRadicals are usually long-lived species.
- EA radical can be neutral, positively charged, or negatively charged.
12.Alkanes are highly reactive towards nucleophiles and electrophiles.
Easy
True or false?
13.Match each term with its correct description.
Medium
- Radical
- Homolytic fission
- Initiation step
- Propagation step
- Termination step
- A species with an unpaired electron
- Bond breaking where each atom takes one electron
- The step where radicals are first formed
- The step where radicals react with molecules to form new radicals
- The step where two radicals combine to form a stable molecule
14.Place the steps of the free radical substitution mechanism in the correct order for the reaction of ethane with chlorine.
Medium
- Cl2 → 2Cl•
- C2H6 + Cl• → C2H5• + HCl
- C2H5• + Cl2 → C2H5Cl + Cl•
- C2H5• + Cl• → C2H5Cl
15.Which equation represents the initiation step?
Medium
- ACH4 + Cl2 → CH3Cl + HCl
- BCl2 → 2Cl•
- CCH4 + Cl• → CH3• + HCl
- DCH3• + Cl2 → CH3Cl + Cl•
16.Which equation represents a propagation step?
Medium
- ACl2 → 2Cl•
- BCH4 + Cl• → CH3• + HCl
- CCH3• + Cl• → CH3Cl
- DCH3• + CH3• → C2H6
17.Which equation represents a termination step?
Medium
- ACH4 + Cl• → CH3• + HCl
- BCH3• + Cl2 → CH3Cl + Cl•
- CCH3• + Cl• → CH3Cl
- DCl2 → 2Cl•
18.Which of the following species contain an unpaired electron and are therefore radicals? (select all that apply)
Medium
- ACl•
- BCl–
- CNO•
- DOH•
- EMg2+
19.Match each radical to its correct classification.
Medium
- Br•
- •C6H6–
- C2H5OH•+
- O2•–
- atomic radical
- anionic radical
- cationic radical
- polyatomic radical
20.Place the stages of the free radical substitution mechanism in the correct sequence for the reaction of ethane with chlorine.
Medium
- Initiation
- Propagation
- Termination
21.Which equation represents a valid propagation step in the free radical substitution of ethane with chlorine?
Medium
- ACH3CH3 + Cl• → CH3CH2• + HCl
- BCH3CH3 + Cl2 → CH3CH2Cl + HCl
- CCH3CH2• + Cl• → CH3CH2Cl
- DCl2 → 2Cl•
22.Which equation represents a possible termination step in the free radical substitution of ethane with chlorine?
Medium
- ACH3CH2• + Cl• → CH3CH2Cl
- BCH3CH3 + Cl• → CH3CH2• + HCl
- CCl2 → 2Cl•
- DCH3CH2• + Cl2 → CH3CH2Cl + Cl•
23.Homolytic fission of a covalent bond produces two radicals.
Easy
True or false?
24.Which type of bond breaking occurs when a chlorine molecule is exposed to UV light in the initiation step of halogenation?
Medium
- AHomolytic fission
- BHeterolytic fission
- CNucleophilic substitution
- DElectrophilic addition
25.Which of the following are reasons why alkanes are relatively unreactive? (select all that apply)
Hard
- AStrong C–C and C–H bonds
- BNon-polar nature of the molecules
- CPresence of a lone pair on carbon
- DLack of electron-deficient or electron-rich areas
- EReadily form ions in solution
26.A reaction scheme is shown: Step 1: Cl2 → 2Cl• Step 2: CH4 + Cl• → •CH3 + HCl Step 3: •CH3 + Cl2 → CH3Cl + Cl• Step 4: Cl• + Cl• → Cl2 Which statement about this scheme is correct?
Medium
- ASteps 1 and 4 are initiation steps
- BStep 1 is initiation and steps 2 and 3 are propagation steps
- CSteps 2 and 3 are termination steps
- DStep 4 is a propagation step because it regenerates chlorine
27.Ethane reacts with excess chlorine under UV light. Assuming complete substitution, what is the overall balanced equation for the formation of hexachloroethane?
Medium
- AC2H6 + 3Cl2 → C2Cl6 + 3H2
- BC2H6 + 6Cl2 → C2Cl6 + 6HCl
- CC2H6 + 3Cl2 → C2Cl6 + 6HCl
- DC2H6 + 6Cl2 → C2Cl6 + 3H2
28.A student writes four equations for the reaction of ethane with chlorine under UV light. Which of the following equations are valid propagation steps? (select all that apply)
Hard
- AC2H6 + Cl• → C2H5• + HCl
- BC2H5• + Cl2 → C2H5Cl + Cl•
- CC2H6 + Cl• → C2H5Cl + H•
- DCl• + Cl• → Cl2
- EC2H5• + Cl• → C2H5Cl
29.In the free radical substitution of ethane with chlorine, the reaction produces a single pure organic product.
Easy
True or false?
30.Match each observation from the reaction of ethane with chlorine under UV light to the correct explanation.
Medium
- The reaction does not occur in the dark.
- A mixture of chlorinated products is formed.
- The reaction stops when two radicals combine.
- UV light provides energy for homolytic fission of Cl–Cl.
- Further substitution of hydrogen atoms by chlorine can occur.
- Two radicals form a single unreactive molecule in a termination step.
31.Place the following steps in the correct order for the free radical substitution of ethane with chlorine under UV light.
Medium
- Cl2 → 2Cl•
- C2H6 + Cl• → C2H5• + HCl
- C2H5• + Cl2 → C2H5Cl + Cl•
- C2H5• + Cl• → C2H5Cl
32.A compound is analysed and found to contain 20.3% carbon, 30.0% chlorine, 48.1% fluorine, and the remainder hydrogen by mass. What is the empirical formula of this compound?
Hard
- AC2H2ClF3
- BCHClF2
- CCH2ClF
- DC2H4ClF
33.Ethane reacts with excess bromine under UV light to form tribromoethane. What is the minimum number of free radical mechanism steps required to produce tribromoethane from ethane?
Hard
- A5
- B6
- C7
- D8
34.In the free-radical substitution of ethane with chlorine, a student writes the equation CH3CH3 + Cl• → CH3CH2Cl + H•. Which statement explains why this equation is incorrect?
Medium
- AA hydrogen radical, H•, is not formed in a propagation step; the hydrogen is removed as HCl
- BChlorine radicals cannot react with ethane because ethane is non-polar
- CThe reaction should produce a chlorine molecule, Cl2, not a hydrogen radical
- DThe equation shows a termination step, not a propagation step
35.In the laboratory, a student mixes hexane with bromine water in a test tube. The mixture is orange. The test tube is then placed in bright sunlight. Which observation shows that a free-radical substitution reaction has occurred?
Medium
- AThe orange colour fades and fumes of hydrogen bromide are produced
- BA white precipitate forms and the solution turns colourless
- CThe mixture separates into two layers, both colourless
- DThe orange colour intensifies and the test tube becomes warm
36.A student wants to prepare a pure sample of chloroethane by reacting ethane with chlorine. Which statement best explains why this method is unsuitable?
Medium
- AThe reaction is slow and requires UV light, so it is not practical
- BA mixture of chlorinated products is formed because substitution can continue
- CEthane does not react with chlorine under any conditions
- DThe reaction produces only chloroethane, but it is difficult to separate from HCl
37.Which experimental condition is essential to initiate the free-radical substitution of ethane with bromine?
Medium
- AUltraviolet light
- BA high pressure of bromine
- CAn aqueous acid catalyst
- DA low temperature
38.Which of the following statements about the termination step in the free-radical substitution of ethane with chlorine are correct? (select all that apply)
Medium
- ATwo radicals combine to form a single unreactive molecule
- BThe chain reaction stops
- CA chlorine radical reacts with ethane to form chloroethane and a hydrogen radical
- DPossible products include butane and chloroethane
- EThe termination step regenerates a chlorine radical
39.In the free-radical substitution of ethane with chlorine, the initiation step requires ultraviolet light.
Easy
True or false?
40.In the propagation step of the chlorination of ethane, a chlorine radical is regenerated.
Medium
True or false?
41.What type of bond breaking occurs in the initiation step of free radical substitution?
Easy
42.In the free radical substitution of ethane with chlorine, how many electrons does a chlorine radical (Cl•) have?
Medium
43.Complete the sentence about the initiation step in the chlorination of ethane.
Medium
In the initiation step, UV light causes the Cl–Cl bond to break by ____ fission, forming two chlorine ____.
44.Complete the equation for the initiation step in the reaction of chlorine with ethane under UV light.
Easy
Cl–Cl → ____
45.What name is given to the step in a free radical substitution mechanism where two radicals combine to stop the chain reaction?
Medium
46.How many electrons does a chlorine radical (Cl•) have?
Medium
47.In the free radical substitution of ethane with chlorine, the initiation step is Cl2 → 2Cl•. Using the bond enthalpy of Cl–Cl = 242 kJ mol⁻¹, calculate the minimum energy, in kJ mol⁻¹, required to break one mole of Cl–Cl bonds.
Medium
48.Complete the equations for the propagation steps in the reaction of ethane with chlorine under UV light.
Medium
C2H6 + Cl• → ____ + HCl C2H5• + Cl2 → ____ + Cl•
49.Name the type of bond breaking that occurs when a chlorine molecule is exposed to UV light in the initiation step of halogenation of alkanes.
Medium
50.How many valence electrons are in a chlorine free radical, Cl•?
Medium
51.How many unpaired electrons are present in a bromine radical, Br•?
Medium
52.How many radicals are produced when one chlorine molecule undergoes homolytic fission?
Medium
53.Define the term free radical and explain why radicals are highly reactive.
Hard
54.Explain why radicals are highly reactive and usually short-lived.
Hard
55.Explain why alkanes are relatively unreactive towards polar reagents such as nucleophiles and electrophiles.
Hard
56.Define the term free radical and explain why radicals are highly reactive and usually short-lived.
Hard
57.Explain why the Cl–Cl bond is broken during the initiation step of the chlorination of methane rather than a C–H bond.
Hard
58.In the free radical substitution of ethane with chlorine, how many electrons does a chlorine radical (Cl•) have?
Medium
- A18 electrons
- B17 electrons
- C16 electrons
- D19 electrons
59.How many electrons does a chlorine radical (Cl•) have?
Medium
- A17 electrons
- B16 electrons
- C18 electrons
- D19 electrons
60.In the free radical substitution of ethane with chlorine, the initiation step is Cl2 → 2Cl•. Using the bond enthalpy of Cl–Cl = 242 kJ mol⁻¹, calculate the minimum energy, in kJ mol⁻¹, required to break one mole of Cl–Cl bonds.
Medium
- A290 kJ mol⁻¹
- B266 kJ mol⁻¹
- C242 kJ mol⁻¹
- D218 kJ mol⁻¹
61.How many valence electrons are in a chlorine free radical, Cl•?
Medium
- A8 electrons
- B7 electrons
- C6 electrons
- D9 electrons
62.How many unpaired electrons are present in a bromine radical, Br•?
Medium
- A3 unpaired electrons
- B1 unpaired electrons
- C0 unpaired electrons
- D2 unpaired electrons
63.How many radicals are produced when one chlorine molecule undergoes homolytic fission?
Medium
- A2 radicals
- B4 radicals
- C1 radicals
- D3 radicals

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