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Questions and Answers
Questions and Answers
Which statement accurately describes the nuanced distinction between the reactivity orders of alcohols in hydrogen halide reactions and the reactivity order of alkyl halides in Finkelstein reactions, considering the underlying mechanistic principles?
Which statement accurately describes the nuanced distinction between the reactivity orders of alcohols in hydrogen halide reactions and the reactivity order of alkyl halides in Finkelstein reactions, considering the underlying mechanistic principles?
- Alcohol reactivity in H-X reactions follows an order ($3^\circ > 2^\circ > 1^\circ$) consistent with S$_N$1 pathway, involving carbocation stability, contrasting with alkyl halide reactivity (RI > RBr > RCl) in Finkelstein, which is governed by the superior nucleophilicity of iodide displacing weaker leaving groups via an S$_N$2 mechanism. (correct)
- The reactivity of alcohols ($3^\circ > 2^\circ > 1^\circ$) in Grove's process mirrors the steric hindrance in S$_N$1-like bond scission, while alkyl halide reactivity (RI > RBr > RCl) in Finkelstein is governed by the leaving group ability of halides in nucleophilic substitution, favouring larger, more polarizable iodides.
- The observed alcohol reactivity ($3^\circ > 2^\circ > 1^\circ$) in H-X reactions is primarily due to the electrophilic character of the hydroxyl oxygen, and Finkelstein reaction reactivity (RCl > RBr > RI) is exclusively influenced by the size of the attacking halide nucleophile.
- Alcohol reactivity ($1^\circ > 2^\circ > 3^\circ$) in H-X reactions is due to inductive effects stabilizing carbocations, whereas alkyl halide reactivity (RF > RCl > RBr > RI) in Finkelstein reactions is dictated by the strength of the C-X bond, with stronger bonds leading to lower reactivity.
The Darzen's process, utilizing thionyl chloride and an alcohol, is universally considered the most superior method for synthesizing all classes of alkyl halides (fluorides, chlorides, bromides, and iodides) due to its highly selective product formation and ease of byproduct removal.
The Darzen's process, utilizing thionyl chloride and an alcohol, is universally considered the most superior method for synthesizing all classes of alkyl halides (fluorides, chlorides, bromides, and iodides) due to its highly selective product formation and ease of byproduct removal.
False (B)
Elaborate on the mechanistic rationale behind the observed reactivity order ($1^\circ > 2^\circ > 3^\circ$) for the alkyl group in the Borodine-Hunsdiecker reaction and explain how this contrasts with typical S$_N$1 reactivity trends.
Elaborate on the mechanistic rationale behind the observed reactivity order ($1^\circ > 2^\circ > 3^\circ$) for the alkyl group in the Borodine-Hunsdiecker reaction and explain how this contrasts with typical S$_N$1 reactivity trends.
The Borodine-Hunsdiecker reaction proceeds via a free radical mechanism involving alkoxy radicals. The reactivity order ($1^\circ > 2^\circ > 3^\circ$) is observed because the primary alkyl radical, being less sterically hindered, is more readily formed and reacts faster in the propagation steps compared to more substituted radicals. This contrasts with S$_N$1 reactions, where reactivity follows $3^\circ > 2^\circ > 1^\circ$ due to the stability of carbocation intermediates.
The oxidation of primary alkyl halides yielding aldehydes and secondary alkyl halides yielding ketones, either by Dimethyl Sulphoxide or Reaction with (CH$_2$)$_6$N$_4$ followed by hydrolysis, is fundamentally limited by the presence of ______ atoms relative to the halogen-bearing carbon.
The oxidation of primary alkyl halides yielding aldehydes and secondary alkyl halides yielding ketones, either by Dimethyl Sulphoxide or Reaction with (CH$_2$)$_6$N$_4$ followed by hydrolysis, is fundamentally limited by the presence of ______ atoms relative to the halogen-bearing carbon.
Match the following dihalide types with their defining structural characteristics:
Match the following dihalide types with their defining structural characteristics:
Consider the comprehensive reactivity hierarchy for alkyl halides: Polarity (RF > RCl > RBr > RI), Boiling Point (RI > RBr > RCl > RF), and General Reactivity (RI > RBr > RCl > RF). Which choice most profoundly explains why the trend in Polarity diverges from Boiling Point and General Reactivity?
Consider the comprehensive reactivity hierarchy for alkyl halides: Polarity (RF > RCl > RBr > RI), Boiling Point (RI > RBr > RCl > RF), and General Reactivity (RI > RBr > RCl > RF). Which choice most profoundly explains why the trend in Polarity diverges from Boiling Point and General Reactivity?
The Friedel-Crafts alkylation of benzene with alkyl halides exclusively exemplifies an electrophilic substitution reaction where the alkyl halide acts as the electrophile, universally undergoing this transformation without exception in its structure.
The Friedel-Crafts alkylation of benzene with alkyl halides exclusively exemplifies an electrophilic substitution reaction where the alkyl halide acts as the electrophile, universally undergoing this transformation without exception in its structure.
Derive the underlying principle for predicting whether an alkane with an odd or even number of carbon atoms can be synthesized via the Wurtz reaction or the mixed Wurtz reaction, considering the combinatorial possibilities of alkyl radicals.
Derive the underlying principle for predicting whether an alkane with an odd or even number of carbon atoms can be synthesized via the Wurtz reaction or the mixed Wurtz reaction, considering the combinatorial possibilities of alkyl radicals.
The 'Williamson ether synthesis' reaction involving an alkyl halide and sodium alkoxide, R-X + NaOR' R-O-R' + NaX, is a quintessential example of an ______ reaction, particularly effective with primary alkyl halides for optimal yields.
The 'Williamson ether synthesis' reaction involving an alkyl halide and sodium alkoxide, R-X + NaOR' R-O-R' + NaX, is a quintessential example of an ______ reaction, particularly effective with primary alkyl halides for optimal yields.
Match the specific reducing conditions with the resulting alkane derivatives from a haloalkane:
Match the specific reducing conditions with the resulting alkane derivatives from a haloalkane:
The industrial synthesis of carbon tetrachloride (CCl$_4$) from carbon disulfide (CS$_2$) involves an initial reaction with chlorine to form CCl$_4$ and sulphur monochloride (S$_2$Cl$_2$), followed by the disproportionation of S$_2$Cl$_2$ in the presence of CS$_2$. Which of the following statements most accurately captures the intricate balance of reagents and conditions necessary for high selectivity and yield in this industrial process?
The industrial synthesis of carbon tetrachloride (CCl$_4$) from carbon disulfide (CS$_2$) involves an initial reaction with chlorine to form CCl$_4$ and sulphur monochloride (S$_2$Cl$_2$), followed by the disproportionation of S$_2$Cl$_2$ in the presence of CS$_2$. Which of the following statements most accurately captures the intricate balance of reagents and conditions necessary for high selectivity and yield in this industrial process?
Despite the polar nature of the carbon-halogen bond, alkyl halides are predominantly soluble in water due to their capacity to form strong hydrogen bonds with water molecules, thereby overcoming the cohesive forces within the water lattice.
Despite the polar nature of the carbon-halogen bond, alkyl halides are predominantly soluble in water due to their capacity to form strong hydrogen bonds with water molecules, thereby overcoming the cohesive forces within the water lattice.
Explain the fundamental chemical principle differentiating the reaction of alkyl cyanides (R-CN) with KCN versus AgCN, leading to divergent product outcomes (nitriles vs. isonitriles), and specify the key mechanistic factors at play.
Explain the fundamental chemical principle differentiating the reaction of alkyl cyanides (R-CN) with KCN versus AgCN, leading to divergent product outcomes (nitriles vs. isonitriles), and specify the key mechanistic factors at play.
The 'Strecker reaction,' involving the reaction of an alkyl halide with sodium sulfite (Na$_2$SO$_3$), is a pivotal method for the synthesis of ______, vital intermediates in the production of detergents.
The 'Strecker reaction,' involving the reaction of an alkyl halide with sodium sulfite (Na$_2$SO$_3$), is a pivotal method for the synthesis of ______, vital intermediates in the production of detergents.
Match the Grignard reagent reactions with the class of alcohol produced from carbonyl compounds:
Match the Grignard reagent reactions with the class of alcohol produced from carbonyl compounds:
In the reduction of polyhalogenated methanes (e.g., CHCl$_3$) by nascent hydrogen, the resulting product (e.g., CH$_4$ from CHCl$_3$) signifies a stepwise dehalogenation process. Which of the following precisely articulates the controlled application of varying equivalents of nascent hydrogen, and the specific reducing agents, to achieve distinct levels of reduction, from dichloromethane to methane?
In the reduction of polyhalogenated methanes (e.g., CHCl$_3$) by nascent hydrogen, the resulting product (e.g., CH$_4$ from CHCl$_3$) signifies a stepwise dehalogenation process. Which of the following precisely articulates the controlled application of varying equivalents of nascent hydrogen, and the specific reducing agents, to achieve distinct levels of reduction, from dichloromethane to methane?
The 'Reimer-Tiemann Formylation' using phenol and chloroform in alkaline medium predominantly yields salicylaldehyde, a reaction that mechanistically involves the formation of a dichlorocarbene intermediate as the effective electrophile.
The 'Reimer-Tiemann Formylation' using phenol and chloroform in alkaline medium predominantly yields salicylaldehyde, a reaction that mechanistically involves the formation of a dichlorocarbene intermediate as the effective electrophile.
Propose a detailed explanation for why only bromo derivatives are typically obtained from the Silver Salt of Carboxylic Acid reaction (Borodine-Hunsdiecker reaction), considering the underlying radical mechanism and the relative stabilities and reactivities of halogen radicals.
Propose a detailed explanation for why only bromo derivatives are typically obtained from the Silver Salt of Carboxylic Acid reaction (Borodine-Hunsdiecker reaction), considering the underlying radical mechanism and the relative stabilities and reactivities of halogen radicals.
The inherent polarity of the carbon-halogen bond notwithstanding, alkyl halides exhibit insolubility in water primarily because they lack the ability to form ______ bonds with water molecules, a critical prerequisite for dissolution.
The inherent polarity of the carbon-halogen bond notwithstanding, alkyl halides exhibit insolubility in water primarily because they lack the ability to form ______ bonds with water molecules, a critical prerequisite for dissolution.
Match the specific reaction conditions for dihalides with the expected product class:
Match the specific reaction conditions for dihalides with the expected product class:
Questions and Answers
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Flashcards
Flashcards
Alkyl Halide
Alkyl Halide
Organic compounds where a halogen atom (F, Br, Cl, I) is directly linked to a carbon atom.
Alkyl Halide General Formula
Alkyl Halide General Formula
The general formula for alkyl halides, where 'n' represents the number of carbon atoms and 'X' represents a halogen (F, Br, Cl, I).
Alkyl Halide Carbon Hybridization
Alkyl Halide Carbon Hybridization
The hybridization state of the carbon atom in alkyl halides to which the halogen is attached.
Monohalides
Monohalides
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Dihalides
Dihalides
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Trihalides
Trihalides
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Tetrahalides
Tetrahalides
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Polyhalides
Polyhalides
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Halogenation of Alkanes
Halogenation of Alkanes
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From Alkene (Hydrohalogenation)
From Alkene (Hydrohalogenation)
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From Alcohol (Using Dry H-X)
From Alcohol (Using Dry H-X)
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Reactivity Order of HX
Reactivity Order of HX
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Grove's Process
Grove's Process
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Darzen's Process
Darzen's Process
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Borodine-Hunsdiecker Reaction
Borodine-Hunsdiecker Reaction
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Finkelstein Reaction
Finkelstein Reaction
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Swarts Reaction
Swarts Reaction
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Physical State of Alkyl Halides
Physical State of Alkyl Halides
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Insolubility in Water
Insolubility in Water
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Reactivity Order of Alkyl Halides
Reactivity Order of Alkyl Halides
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Study Notes
Study Notes
Characteristics of Haloalkanes
- Haloalkanes are organic compounds where a halogen atom is directly bonded to a carbon atom.
- Their general formula is CnH2n+1X, where X can be Fluorine (F), Bromine (Br), Chlorine (Cl), or Iodine (I).
- The carbon atom involved in the bond has sp³ hybridization, resulting in a tetrahedral geometry and a bond angle of 109° 28'.
- Based on the number of halogen atoms, alkyl halides are classified as monohalides (one halogen atom), dihalides (two halogen atoms), trihalides (three halogen atoms), tetrahalides (four halogen atoms), and polyhalides (more than four halogen atoms).
- Alkyl halides can exhibit chain and position isomerism, and if an unsymmetrical or chiral carbon is present, they also show optical isomerism.
Methods of Preparation of Alkyl Halides
- Halogenation of Alkanes: This process occurs via a free radical mechanism.
- R-H + X-X (with light, hν) → R-X + H-X
- R-H + Cl-SO₂-Cl (with peroxide) → R-Cl + SO₂ + H-Cl
- From Alkene (Hydrohalogenation): Adding HX across a double bond.
- R-CH=CH-H + H-X → R-CH-CH (where X attaches to one carbon and H to the other carbon of the original double bond).
- Monohaloalkanes cannot be obtained from alkynes using this method.
- From Alcohols:
- Using dry HX: R-OH + H-X (with Anhydrous ZnCl₂ and heating to 300°C) → R-X + H₂O. This is known as Grove's Process.
- The reactivity order of HX is HI > HBr > HCl.
- The reactivity order of alcohols is 3° > 2° > 1° > MeOH.
- Using PCl₃: 3ROH + PCl₃ → 3R-Cl + H₃PO₃.
- Using PCl₅: ROH + PCl₅ → R-Cl + HCl + POCl₃.
- Bromine or Iodine derivatives are not typically obtained from these reactions due to the instability of PBr₅ or PI₅ with their larger size.
- Darzen's Process (Best Method): ROH + SOCl₂ (with Pyridine) → RCl + HCl↑ + SO₂↑.
- Using dry HX: R-OH + H-X (with Anhydrous ZnCl₂ and heating to 300°C) → R-X + H₂O. This is known as Grove's Process.
- From Silver Salt of Carboxylic Acid (Borodine - Hunsdiecker's Reaction): This method primarily yields bromo derivatives through a free radical mechanism.
- R-C(=O)-O-Ag + Br-Br (with CCl₄) → R-Br + CO₂↑ + AgBr↓.
- The reactivity of the alkyl group in this reaction follows the order: 1° > 2° > 3°.
- This is also an example of decarboxylation.
- From Alkyl Halide (Halogen Exchange Reactions):
- Finkelstein Reaction: R-Br or R-Cl + KI (with Acetone) → R-I + KCl. This reaction involves the exchange of halogens.
- Swarts Reaction: R-Br or R-Cl (with AgF or Hg₂F₂) → R-F.
Physical Properties of Alkyl Halides
- Alkyl halides are generally colorless oily liquids with a sweet or pleasant smell.
- CH₃F, CH₃Cl, CH₃-CH₂-F, and CH₃-CH₂-Cl are gaseous at room temperature.
- Alkyl halides with 18 or more carbon atoms exist as solids.
- Carbon-halogen bonds are polar, but alkyl halides are insoluble in water as they cannot form hydrogen bonds with water.
- They are soluble in organic solvents.
- Melting and boiling points (M.P. & B.P.) are directly proportional to molecular weight. For the same alkyl group, the order is RI > RBr > RCl > RF.
- The polarity order is RF > RCl > RBr > RI.
- The reactivity order is RI > RBr > RCl > RF.
- For the same halide group, the reactivity order is 3° halide > 2° halide > 1° halide.
- Fluorides and chlorides are lighter than water, while bromides and iodides are heavier due to greater density. CH₂I₂ is a heavier liquid than mercury.
Chemical Properties of Alkyl Halides
Oxidation Reaction
- Only primary and secondary alkyl halides undergo oxidation; tertiary alkyl halides do not.
- Primary alkyl halides yield aldehydes, while secondary alkyl halides yield ketones.
- Common oxidizing agents include dimethyl sulfoxide (DMSO) or reaction with (CH₂)₆N₄ followed by hydrolysis.
- Reactivity depends on the number of α-hydrogens.
- Oxidation of Benzyl halides by (CH₂)₆N₄ is known as Sommelet aldehyde synthesis.
- Oxidation of alkyl halides with DMSO is known as Swern oxidation.
Reduction
- Haloalkanes can be reduced to alkanes.
- R-X + 2H → R-H + HX.
- Using nascent hydrogen: Liberated from Na/C₂H₅OH, Sn/HCl, Zn/HCl, or Zn-Cu couple/C₂H₅OH.
- Using hydride ion [:H⁻]: Liberated from LiAlH₄ or NaBH₄; this is a nucleophilic substitution reaction.
- Catalytic hydrogenation: R-X + H₂ (with Pd catalyst) → R-H + HX.
- Reduction of RI with HI: R-X + HI (with red P and at 150°C) → R-H + I₂.
Reacti+on with KOH
- Aqueous KOH: R-X + KOH(aq.) → R-OH + K-X.
- Alcoholic KOH: Dehydrohalogenation occurs, forming alkenes.
- R-CH₂-CH₂-X + KOH(alc.) → R-CH=CH₂.
Reaction with KCN
- R-X + KCN → R-CN + KX (Alkane nitrile).
- Alkane nitriles yield various products:
- Complete hydrolysis: R-C≡N (with H₂O/H⁺, complete hydrolysis) → R-C(=O)-OH + NH₃.
- Partial hydrolysis: R-C≡N (with H₂O, partial hydrolysis) → R-C(=O)-NH₂.
- Reduction: R-C≡N (with LiAlH₄) → R-CH₂-NH₂ (Primary amine).
Reaction with AgCN
- R-X + AgCN → R-N=C + AgX.
- Hydrolysis of R-N=C: R-N=C (hydrolysis) → R-NH₂ + HCOOH.
- Reduction of R-N=C: R-N=C (reduction) → R-NH-CH₃ (Secondary amine).
Reaction with KNO₂
- R-X + K-O-N=O → R-O-N=O (Alkyl nitrite).
Reaction with AgNO₂
- R-X + Ag-O-N=O → R-NO₂ (Nitro alkane).
Reaction with KSH
- R-X + K-SH → R-SH (Alkane thiol).
Reaction with Na₂S
- 2R-X + Na₂S → R-S-R (Dialkyl sulphide).
Reaction with Na₂SO₃
- R-X + Na₂SO₃ → RSO₃Na + NaX (Alkyl sodium sulphonate). This is known as Strecker reaction.
Reaction with NaOR (Williamson Ether Synthesis)
- R-X + NaOR → R-O-R + NaX. This reaction forms ethers.
Reaction with Ag₂O
- Using dry Ag₂O: 2R-X + Ag₂O → R-O-R + 2AgX.
- Using moist Ag₂O: 2R-X + Ag₂O + H₂O → R-OH + 2AgX.
Reaction with Silver Acetate (Esterification)
- R-X + Ag-O-C-CH₃ → R-O-C-CH₃ (Ester).
Coupling Reactions
Wurtz Reaction
- An alkane with an even number of carbon atoms can be obtained.
- R-X + 2Na + X-R (with dry ether) → R-R + 2NaX.
- Alkanes with an odd number of carbon atoms can be obtained by a mixed Wurtz Reaction.
- R-X + X-R' (with dry ether/Na) → R-R + R-R' + R'-R'.
Wurtz-Fitting Reaction
- R-I + 2Na + I-Ar (with dry ether) → R-Ar + 2NaI.
- Example: CH₃-I + 2Na + I-C₆H₅ (with dry ether) → CH₃-C₆H₅ + 2NaI.
Reaction with Metals
With Na (Wurtz reaction)
- R-X + 2Na + X-R (with dry ether) → R-R + 2NaX.
With Mg (Grignard reaction)
- R-X + Mg (with dry ether) → R-Mg-X.
- The reactivity order of halides with Mg is RI > RBr > RCl.
- Grignard reagents form stable complexes with ether solvents.
With Zn dust (Frankland reaction)
- R-X + 2Zn + X-R → R-Zn-R + ZnX₂ (Dialkyl zinc).
- Dialkyl zinc is known as 'Frankland Reagent'.
With Li
- R-Cl + 2Li (with dry ether) → RLi + LiCl (Alkyl lithium).
- Alkyl lithium is more reactive than Grignard reagent.
With Na-lead alloy
- 4CH₃-CH₂-Cl + 4Na-Pb → (CH₃-CH₂)₄Pb + 4NaCl (Tetraethyl lead, TEL).
- TEL is used as an antiknocking agent.
Reaction with Benzene (Friedel-Crafts Reaction)
- Benzene + R-X (with AlCl₃ at 180°C) → R-Benzene + H-X.
- Alkyl halides show electrophilic substitution reactions in this context, which is an exception.
Uses of Alkyl Halides
- Alkyl halides act as weak refrigerants, though freons are more common now.
- They are used in the synthesis of detergents via the Strecker reaction.
- They are used in the production of antiknock compounds.
- Alkyl bromides and iodides are vital for synthesizing various organic compounds in labs and industry.
- They serve as starting materials for manufacturing alcohols, ethers, and esters.
- Important organometallic compounds like Grignard's reagents and Frankland's reagents are synthesized from alkyl halides.
Dihalides
Types of Dihalides
- Gem dihalide: Two identical halogen atoms attached to the same carbon atom.
- Vicinal dihalide: Two identical halogen atoms attached to adjacent carbon atoms.
Methods of Preparation of Gem Dihalides
- From Alkyne (Hydrohalogenation):
- R-C≡C-H + HX → R-C=C-H (followed by another HX addition) → R-CH₃.
- From Carbonyl Compounds:
- RCHO + PCl₅ → R-CHCl₂ + POCl₃ (if a ketone is used, an internal dihalide is formed).
Methods of Preparation of Vicinal Dihalides
- From Alkene (Halogenation):
- R-CH=CH₂ + Cl₂ → R-CH-CH₂ (where one Cl attaches to each carbon of the original double bond).
- From Vicinal Glycol:
- R-CH-OH-CH₂-OH + PCl₅ → R-CH-Cl-CH₂-Cl + 2HCl + 2POCl₃.
Physical Properties of Dihalides
- Dihalides are colorless, pleasant-smelling liquids, insoluble in water, but soluble in organic solvents.
- Melting and boiling points correlate directly with molecular mass; vicinal dihalides have higher melting/boiling points than gem dihalides.
- Vicinal dihalides are more reactive than gem dihalides but less reactive than monohalides.
Chemical Properties of Dihalides
- Reaction with aqueous KOH:
- R-CH-X-CH₂-X + KOH(aq.) → R-CH-OH-CH₂-OH (Glycol formation).
- Reaction with alcoholic KOH:
- R-CH₂-CH-X-CH-X (with Alc. KOH) → R-C≡CH (Alkyne formation).
- Reaction with Zinc dust:
- Gem dihalides react with Zn dust to form higher symmetrical alkenes.
- Vicinal dihalides react with Zn dust to form respective alkenes.
- α,ω-dihalides form cyclic alkanes.
- Reaction with KCN:
- R-CH-X-CH₂-X + KCN → R-CH-CN-CH₂-CN (Dinitrile formation).
- Subsequent hydrolysis forms di-carboxylic acids, while reduction forms di-amines.
- Other substitution reactions: Dihalides can lead to the formation of ethylene amines or esters through substitution.
Trihalides
Definition
- Trihalo derivatives of alkanes are known as trihalides.
- The method of preparing trihalides is called the haloform reaction.
Preparation of Haloform (CHX₃)
- Haloform reaction can occur with compounds containing the CH₃CO- group (like methyl ketones, acetaldehyde) or CH₃CH(OH)- group (like ethyl alcohol, 2-alkanols) when heated with alkali and halogen.
- Example: C₂H₅OH + 4X₂ + 6NaOH → CHX₃ + 5NaX + 5H₂O + HCOONa.
- Example: CH₃COCH₃ + 3X₂ + 4NaOH → CHX₃ + 3NaX + CH₃COONa + H₂O.
Chloroform (CHCl₃)
Preparation of Chloroform
- Laboratory Methods - Chloroform Reaction:
- By heating ethyl alcohol with bleaching powder, a multi-step reaction occurs:
- CaOCl₂ + H₂O → Ca(OH)₂ + 2Cl⁻
- CH₃CH₂OH + 2Cl⁻ → CH₃CHO + 2HCl
- CH₃CHO + 6Cl⁻ → CCl₃CHO (Chloral) + 3HCl
- 2CCl₃CHO + Ca(OH)₂ → 2CHCl₃ + (HCOO)₂Ca
- By heating ethyl alcohol with bleaching powder, a multi-step reaction occurs:
- From Chloral: Chloral (CCl₃CHO) reacts with chlorobenzene in the presence of conc. H₂SO₄ to form DDT (Dichloro Diphenyl Trichloro ethane).
- Preparation of pure Chloroform: Alkaline solution of chlorohydrate upon distillation yields pure chloroform.
- Preparation of trihalide using 'Pyrene': CCl₄ + 2H (with Fe/H₂O reduction) → CHCl₃ + HCl.
Physical Properties of Chloroform
- Chloroform is a colorless liquid with a pleasant smell.
- It is insoluble in water but soluble in organic solvents.
- Chloroform vapors are poisonous and can cause temporary unconsciousness, historically used as an anesthetic.
- Boiling point of CHCl₃ is 61°C.
- It is an excellent solvent for fats, oil, and wax.
- Iodoform is a yellow crystalline solid with a melting point of 119°C.
Chemical Properties of Chloroform
Oxidation
- In the presence of light, chloroform oxidizes with atmospheric oxygen or air to form poisonous phosgene (COCl₂).
- CHCl₃ + 1/2 O₂ (with light) → COCl₂ + HCl.
- To prevent phosgene formation, chloroform is stored in dark, colored bottles, filled completely.
- 0.5 to 1% ethanol solution can be added to convert poisonous phosgene into non-poisonous diethyl carbonate.
- Silver nitrate solution is used to test for phosgene impurity; it forms a white precipitate of AgCl with HCl.
Reaction with HNO₃
- Chloroform reacts with HNO₃ to form chloropicrin (tear gas).
Reaction with Acetone
- Chloroform reacts with acetone to produce chloretone, which is used as a hypnotic agent.
Reaction with Primary Amine (Hoffman Carbylamine Reaction / Isocyanide Test)
- RNH₂ + CHCl₃ + KOH(alc.) → R-N=C + KCl + H₂O.
- The product (isocyanide) has an offensive smell. This reaction is used to test for primary amines, with dichloro carbene being the reacting species.
Reaction with Phenol (Reimer-Tiemann Formylation)
- Phenol + CHCl₃ + KOH(aq) → Salicylaldehyde (o-hydroxy benzaldehyde).
Reaction with 2-Butene
- This reaction involves the formation of 2-methyl butanoic acid.
Reaction with Aqueous NaOH
- CHCl₃ + 3NaOH(aq) → H-C(=O)-OH + 2NaCl + H₂O. (Unstable intermediate leads to formate salt).
Reaction with Silver Powder (Dehalogenation)
- CHX₃ + 6Ag (at high temperature) → CH≡CH + 6AgX.
Reduction
- CHCl₃ can be reduced sequentially:
- With 2H from Zn/HCl: CHCl₃ → CH₂Cl₂ + HCl.
- With 4H from Zn/HCl: CHCl₃ → CH₃Cl + 2HCl.
- With 6H from Zn/H₂O: CHCl₃ → CH₄ + 3HCl.
Uses of Chloroform
- An anesthetic.
- Solvent for fats, oil, and non-polar substances.
- Antiseptic.
- Used in the manufacture of chloretone (hypnotic drug).
- Used in the manufacture of chloropicrin (war gas).
- Used in the manufacture of triphenylmethane dyes.
- Used in the manufacture of Teflon (polymer).
Iodoform (CHI₃)
Iodoform Reaction (Iodoform Test)
- A yellow precipitate of CHI₃ is formed when a compound that undergoes the haloform reaction is mixed with a saturated solution of sodium carbonate and heated with iodine.
- Na₂CO₃ is a strong base due to the hydrolysis of CO₃²⁻ ion.
- Example: 2-Hydroxy 2° alcohol + NaOI → Ketone + NaI + H₂O.
- Methyl ketone + 3NaOI → Triiodo derivative (e.g., CHI₃) + 3NaOH.
- Triiodo derivative + NaOH → CHI₃ + COONa.
Tetrahalide ('Pyrene')
General Method of Preparation
From CS₂
- CS₂ + 3Cl-Cl (at 500°C, with Fe dust-I₂) → CCl₄ + S₂Cl₂ (Sulfur monochloride).
- 2S₂Cl₂ + CS₂ → CCl₄ + 6S↓. This reaction is used for the industrial production of CCl₄.
From CH₄
- CH₄ + Cl₂ → CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄ (sequential chlorination).
From CHCl₃
- CHCl₃ + Cl-Cl (with ultraviolet light) → CCl₄ + HCl.
Physical Properties
- Carbon tetrachloride (CCl₄) is a colorless liquid with a specific smell.
- It is insoluble in water but soluble in organic solvents.
- It is the only non-combustible organic solvent, used as a fire extinguisher known as 'Pyrene'.
Chemical Properties
Reaction with Hot H₂O or Water Vapour
- CCl₄ + H₂O(g) → COCl₂ (poisonous gas 'Phosgene') + 2HCl.
Reaction with Aqueous or Alcoholic KOH
- CCl₄ + 4KOH(aq.) → C(OH)₄ (unstable) → CO₂ → K₂CO₃ + H₂O.
Reaction with Phenol
- CCl₄ reacts with phenol to form salicylic acid. This reaction is called Reimer-Tiemann Carboxylation.
Reaction with Benzene
- 2 Benzene + CCl₄ (with anhy. AlCl₃) → Di-chloro diphenyl methane + 2HCl.
Freons
Definition
- Freons are polychlorofluoro derivatives of alkanes.
Preparation of Freons
- CCl₄ + HF (with SbCl₅) → CCl₃F + HCl.
- C₂Cl₆ + 2HF (with SbCl₅) → C₂F₂Cl₄ + 2HCl (Freons-112).
Nomenclature of Freons
- The common name for freons is "Freon-cba" or "Freon C-1, H+1, F".
- 'c' = number of carbon atoms - 1.
- 'b' = number of hydrogen atoms + 1.
- 'a' = total number of fluorine atoms.
- Example: CFCCl₃ nomenclature translates to Freon-11 (c=0, b=1, a=1).
Properties and Uses of Freons
- Freons are colorless, odorless, unreactive, and non-combustible liquids.
- They have very low boiling points (e.g., CF₂Cl₂ = -29.8°C) and easily convert from gas to liquid, making them suitable as coolants in ACs and refrigerators.
- Used as aerosol propellants in airplanes and rockets.
- Also used as solvents.
- Note: CFCs (chlorofluorocarbons) are a primary cause of ozone layer depletion.
Grignard Reagents
Discovery
- Organomagnesium halides were discovered by French chemist Victor Grignard in 1900.
Preparation of Grignard Reagents (GR.)
- GR. are prepared by reacting an organic halide (RX) with magnesium (Mg) in a dry ether solvent.
- RX + Mg (with dry ether) → RMgX.
- ArX + Mg (with dry ether) → ArMgX.
- The reactivity order of halides with Mg is RI > RBr > RCl.
- GR. form a complex with ether solvent, which provides stability to the reagent.
- This method, usable for 1°, 2°, and 3° alcohols, is less practiced as alkyl halides can directly be converted to corresponding alcohols.
Reactions of Grignard Reagents
Reaction with Carbonyl Compounds
- GR. react with carbonyl compounds to yield 1°, 2°, and 3° alcohols.
- Formaldehyde (methanal, HCHO): Gives 1° alcohol.
- Other aldehydes: Gives 2° alcohols.
- Ketones: Gives 3° alcohols.
Reaction with Esters
- Two moles of GR. react with esters to yield 3° alcohols with two identical alkyl groups corresponding to the alkyl portion of the GR.
- One mole of GR. reacts with esters to form ketones.
- Ketones are more reactive towards GR. than esters, so the ketone formed further reacts with a second GR. molecule.
Reaction with Dialkyl Carbonate
- Preparation of 3° alcohol containing three identical alkyl groups.
- Achieved by reacting 3 moles of GR. with 1 mole of diethyl carbonate.
Reaction with Alkanoyl Halide
- Two moles of GR. react with acid halides (R-C(=O)-X) to give 3° alcohols.
- One mole of GR. reacts with acid halides to form ketones.
- Since ketones are more reactive than acid halides, the formed ketone reacts with a second GR. molecule to produce a 3° alcohol with two identical alkyl groups.
- GR. reaction with formyl halides or methanoyl halides (H-C(=O)-X) gives 2° alcohols with two identical alkyl groups corresponding to the alkyl portion of the GR.
Reaction of Dialkyl Cadmium (R₂Cd) or Dialkyl Lithium Cuprate with Acid Halides
- R₂Cd or dialkyl lithium cuprates react with acid halides (R-C(=O)-X) to yield ketones.
- They react with formyl halides (H-C(=O)-X) to yield aldehydes.
Reaction with Anhydride
- Two moles of GR. react with acid anhydride (R-C(=O)-O-C(=O)-R) to yield 3° alcohol.
- Acid anhydrides react similarly to esters and acid halides with RMgX.
Reaction of RMgX (GR.) with Oxiranes (Epoxides) and Other Cyclic Ethers
- RMgX reacts with oxiranes or cyclic ethers via an SN² mechanism.
- The electron-rich 'R' (nucleophile) of RMgX attacks the partially charged carbon atom of the highly strained oxirane ring, causing the ring to open and form a salt of 1°C alcohol. Acidification of this salt then yields the alcohol.
Reaction with O₂
- GR. react with O₂ to give 1° alcohols.
Reaction with Acids
- RMgX reacts with acids to yield an alkane (R-H) and MgX₂.
- R-MgX + H-X → R-H + MgX₂.
Reaction with R-CN
- RMgX reacts with R-CN to give a ketone.
- The formed ketone can further react with RMgX to yield a 3° alcohol.
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