chemistry-english-note/contents/reactions.tex

\section{Reactions}

\subsection{Types of reactions}

\subsubsection{Combination reaction (Synthesis reaction)}

\textbf{Definition:} Two or more substances combine to form a single product.

\textbf{General form:} \ce{A + B -> AB}

\textbf{Examples:}
\begin{itemize}
\item Formation of water: \ce{2H2 + O2 -> 2H2O}
\item Formation of ammonia: \ce{N2 + 3H2 -> 2NH3}
\item Metal oxide formation: \ce{2Mg + O2 -> 2MgO}
\item Salt formation: \ce{2Na + Cl2 -> 2NaCl}
\end{itemize}

\subsubsection{Decomposition reaction}

\textbf{Definition:} A single compound breaks down into two or more simpler substances.

\textbf{General form:} \ce{AB -> A + B}

\textbf{Examples:}
\begin{itemize}
\item Thermal decomposition of calcium carbonate: \ce{CaCO3 ->[heat] CaO + CO2 ^}
\item Electrolysis of water: \ce{2H2O ->[electrolysis] 2H2 ^ + O2 ^}
\item Decomposition of hydrogen peroxide: \ce{2H2O2 ->[MnO2] 2H2O + O2 ^}
\item Decomposition of potassium chlorate: \ce{2KClO3 ->[heat] 2KCl + 3O2 ^}
\end{itemize}

\subsubsection{Displacement reaction (Substitution reaction)}

\textbf{Definition:} One element replaces another element in a compound.

\textbf{General form:} \ce{A + BC -> AC + B}

\textbf{Examples:}
\begin{itemize}
\item Zinc displacing hydrogen: \ce{Zn + 2HCl -> ZnCl2 + H2 ^}
\item Chlorine displacing bromine: \ce{Cl2 + 2NaBr -> 2NaCl + Br2}
\item Magnesium displacing copper: \ce{Mg + CuSO4 -> MgSO4 + Cu}
\item Iron displacing copper: \ce{Fe + CuSO4 -> FeSO4 + Cu}
\end{itemize}

\subsubsection{Double displacement reaction (Metathesis reaction)}

\textbf{Definition:} Exchange of ions between two compounds.

\textbf{General form:} \ce{AB + CD -> AD + CB}

\textbf{Examples:}
\begin{itemize}
\item Precipitation: \ce{AgNO3 + NaCl -> AgCl v + NaNO3}
\item Neutralization: \ce{HCl + NaOH -> NaCl + H2O}
\item Formation of barium sulfate: \ce{BaCl2 + H2SO4 -> BaSO4 v + 2HCl}
\end{itemize}

\subsubsection{Redox reaction (Oxidation-reduction reaction)}

\textbf{Definition:} Transfer of electrons between species, involving change in oxidation states.

\textbf{Oxidation:} Loss of electrons, increase in oxidation number.

\textbf{Reduction:} Gain of electrons, decrease in oxidation number.

\textbf{Examples:}
\begin{itemize}
\item Combustion: \ce{C + O2 -> CO2}
\item Permanganate oxidation: \ce{2KMnO4 + 5H2C2O4 + 3H2SO4 -> K2SO4 + 2MnSO4 + 10CO2 ^ + 8H2O}
\item Dichromate oxidation: \ce{K2Cr2O7 + 14HCl -> 2KCl + 2CrCl3 + 3Cl2 ^ + 7H2O}
\item Zinc-copper cell: \ce{Zn + Cu^2+ -> Zn^2+ + Cu}
\end{itemize}

\subsubsection{Acid-base reaction (Neutralization)}

\textbf{Definition:} Reaction between an acid and a base to produce salt and water.

\textbf{General form:} \ce{Acid + Base -> Salt + Water}

\textbf{Examples:}
\begin{itemize}
\item Strong acid-strong base: \ce{HCl + NaOH -> NaCl + H2O}
\item Weak acid-strong base: \ce{CH3COOH + NaOH -> CH3COONa + H2O}
\item Dibasic acid: \ce{H2SO4 + 2KOH -> K2SO4 + 2H2O}
\item Carbonate with acid: \ce{Na2CO3 + 2HCl -> 2NaCl + H2O + CO2 ^}
\end{itemize}

\subsubsection{Precipitation reaction}

\textbf{Definition:} Formation of an insoluble solid (precipitate) from aqueous solutions.

\textbf{Examples:}
\begin{itemize}
\item Silver chloride: \ce{Ag+ + Cl- -> AgCl v} (white precipitate)
\item Lead iodide: \ce{Pb^2+ + 2I- -> PbI2 v} (yellow precipitate)
\item Iron(III) hydroxide: \ce{Fe^3+ + 3OH- -> Fe(OH)3 v} (brown precipitate)
\item Copper(II) hydroxide: \ce{Cu^2+ + 2OH- -> Cu(OH)2 v} (blue precipitate)
\end{itemize}

\subsection{Organic reactions}

\subsubsection{Halogenation reaction}

\textbf{Definition:} Introduction of halogen atoms (F, Cl, Br, I) into organic molecules.

\textbf{Free radical halogenation (alkanes):}
\begin{itemize}
\item Chlorination of methane: \ce{CH4 + Cl2 ->[UV light] CH3Cl + HCl}
\item Further substitution: \ce{CH3Cl + Cl2 -> CH2Cl2 + HCl}
\item Bromination of ethane: \ce{C2H6 + Br2 ->[UV light] C2H5Br + HBr}
\end{itemize}

\textbf{Electrophilic halogenation (aromatic):}
\begin{itemize}
\item Bromination of benzene: \ce{C6H6 + Br2 ->[FeBr3] C6H5Br + HBr}
\item Chlorination of benzene: \ce{C6H6 + Cl2 ->[AlCl3] C6H5Cl + HCl}
\item Iodination of benzene: \ce{C6H6 + I2 ->[HNO3] C6H5I + HI}
\end{itemize}

\textbf{Addition halogenation (alkenes):}
\begin{itemize}
\item Bromination of ethene: \ce{C2H4 + Br2 -> C2H4Br2}
\item Test for unsaturation: Decolorization of bromine water
\item Chlorination of propene: \ce{C3H6 + Cl2 -> C3H6Cl2}
\end{itemize}

\subsubsection{Nucleophilic substitution}

\textbf{Definition:} Replacement of a leaving group by a nucleophile.

\textbf{S$_{\mathbf{N}}$1 mechanism (unimolecular):}
\begin{itemize}
\item Two-step process via carbocation intermediate
\item Rate depends only on substrate concentration
\item Favored by tertiary halides and polar solvents
\item Example: \ce{(CH3)3CBr + H2O -> (CH3)3COH + HBr}
\end{itemize}

\textbf{S$_{\mathbf{N}}$2 mechanism (bimolecular):}
\begin{itemize}
\item One-step process with backside attack
\item Rate depends on both substrate and nucleophile concentration
\item Favored by primary halides and aprotic solvents
\item Inversion of configuration (Walden inversion)
\item Example: \ce{CH3Br + OH- -> CH3OH + Br-}
\end{itemize}

\textbf{Common nucleophiles:}
\begin{itemize}
\item Hydroxide (OH$^-$): Forms alcohols
\item Alkoxide (RO$^-$): Forms ethers
\item Cyanide (CN$^-$): Forms nitriles
\item Ammonia (NH$_3$): Forms amines
\item Water (H$_2$O): Forms alcohols (weak nucleophile)
\end{itemize}

\subsubsection{Nucleophilic addition}

\textbf{Definition:} Addition of a nucleophile to a carbonyl group.

\textbf{Examples:}
\begin{itemize}
\item Cyanohydrin formation: \ce{CH3CHO + HCN -> CH3CH(OH)CN}
\item Grignard addition: \ce{CH3MgBr + CH2O -> CH3CH2OH} (after hydrolysis)
\item Bisulfite addition: \ce{RCHO + NaHSO3 -> RCH(OH)SO3Na}
\end{itemize}

\subsubsection{Electrophilic addition}

\textbf{Definition:} Addition of an electrophile to a multiple bond.

\textbf{Examples:}
\begin{itemize}
\item Hydrohalogenation: \ce{C2H4 + HBr -> C2H5Br}
\item Markovnikov's rule: H adds to carbon with more H atoms
\item Hydration: \ce{C2H4 + H2O ->[H+] C2H5OH}
\item Addition of sulfuric acid: \ce{C2H4 + H2SO4 -> C2H5OSO3H}
\end{itemize}

\subsubsection{Elimination reaction}

\textbf{Definition:} Removal of atoms or groups to form multiple bonds.

\textbf{Dehydrohalogenation (E1 and E2):}
\begin{itemize}
\item E2 mechanism: \ce{C2H5Br + KOH ->[alcohol, heat] C2H4 ^ + KBr + H2O}
\item Zaitsev's rule: Major product is more substituted alkene
\item E1 mechanism: Two-step via carbocation
\end{itemize}

\textbf{Dehydration of alcohols:}
\begin{itemize}
\item Ethanol dehydration: \ce{C2H5OH ->[H2SO4, heat] C2H4 ^ + H2O}
\item Intramolecular (forms alkene) vs intermolecular (forms ether)
\end{itemize}

\subsubsection{Oxidation reactions}

\textbf{Oxidation of alcohols:}
\begin{itemize}
\item Primary to aldehyde: \ce{RCH2OH ->[oxidation] RCHO ->[oxidation] RCOOH}
\item Secondary to ketone: \ce{R2CHOH ->[oxidation] R2CO}
\item Tertiary alcohols: Resistant to oxidation
\item Oxidizing agents: \ce{K2Cr2O7/H2SO4}, \ce{KMnO4}, \ce{CrO3}
\end{itemize}

\textbf{Oxidation of aldehydes:}
\begin{itemize}
\item To carboxylic acid: \ce{RCHO ->[oxidation] RCOOH}
\item Mild oxidizing agents work (e.g., Tollens' reagent, Fehling's reagent)
\end{itemize}

\subsubsection{Reduction reactions}

\textbf{Reduction of carbonyl compounds:}
\begin{itemize}
\item Aldehyde to primary alcohol: \ce{RCHO ->[reduction] RCH2OH}
\item Ketone to secondary alcohol: \ce{R2CO ->[reduction] R2CHOH}
\item Reducing agents: \ce{LiAlH4}, \ce{NaBH4}, \ce{H2/Pt}
\end{itemize}

\textbf{Reduction of carboxylic acids:}
\begin{itemize}
\item To primary alcohol: \ce{RCOOH ->[LiAlH4] RCH2OH}
\item Requires strong reducing agent
\end{itemize}

\subsubsection{Condensation reactions}

\textbf{Definition:} Combination of molecules with elimination of small molecule (usually water).

\textbf{Esterification:}
\begin{itemize}
\item Fischer esterification: \ce{RCOOH + R-OH <=>[H+] RCOOR + H2O}
\item Reversible reaction, equilibrium can be shifted
\end{itemize}

\textbf{Aldol condensation:}
\begin{itemize}
\item Self-condensation of aldehydes: \ce{2CH3CHO ->[OH-] CH3CH(OH)CH2CHO}
\item Followed by dehydration: \ce{CH3CH(OH)CH2CHO -> CH3CH=CHCHO + H2O}
\end{itemize}

\subsubsection{Hydrolysis reactions}

\textbf{Ester hydrolysis:}
\begin{itemize}
\item Acidic: \ce{RCOOR + H2O <=>[H+] RCOOH + R-OH}
\item Basic (saponification): \ce{RCOOR + NaOH -> RCOONa + R-OH}
\end{itemize}

\textbf{Amide hydrolysis:}
\begin{itemize}
\item Acidic: \ce{RCONH2 + H2O + HCl ->[heat] RCOOH + NH4Cl}
\item Basic: \ce{RCONH2 + NaOH ->[heat] RCOONa + NH3 ^}
\end{itemize}

\subsection{Named reactions and tests}

\subsubsection{Silver mirror reaction (Tollens' test)}

\textbf{Purpose:} Test for aldehydes; distinguishes aldehydes from ketones.

\textbf{Reagent:} Tollens' reagent - ammoniacal silver nitrate solution \ce{[Ag(NH3)2]+}

\textbf{Principle:} Aldehydes are oxidized to carboxylic acids while silver ions are reduced to metallic silver, forming a silver mirror on the test tube.

\textbf{Preparation of reagent:}
\begin{itemize}
\item \ce{AgNO3 + NaOH -> AgOH v + NaNO3}
\item \ce{AgOH + 2NH3 -> [Ag(NH3)2]OH} (soluble complex)
\end{itemize}

\textbf{Reaction with aldehyde:}
\begin{itemize}
\item \ce{RCHO + 2[Ag(NH3)2]+ + 2OH- -> RCOO- + 2Ag v + 4NH3 + H2O}
\item Formaldehyde: \ce{HCHO + 4[Ag(NH3)2]+ + 4OH- -> CO3^2- + 4Ag v + 8NH3 + 2H2O}
\item Glucose (reducing sugar): \ce{C6H12O6 + 2[Ag(NH3)2]+ + 2OH- -> C6H12O7 + 2Ag v + 4NH3}
\end{itemize}

\textbf{Observation:} Silver mirror forms on the inner surface of the test tube (positive test).

\textbf{Note:} Ketones do not give this reaction. Some $\alpha$-hydroxy ketones may give weakly positive results.

\subsubsection{Fehling's test (Benedict's test)}

\textbf{Purpose:} Test for reducing sugars and aldehydes.

\textbf{Reagent:} Fehling's solution (mixture of Fehling's A and B)
\begin{itemize}
\item Fehling's A: Copper(II) sulfate solution \ce{CuSO4}
\item Fehling's B: Alkaline sodium potassium tartrate solution (Rochelle salt)
\end{itemize}

\textbf{Principle:} Aldehydes reduce Cu$^{2+}$ (blue) to Cu$_2$O (red-brown precipitate).

\textbf{Reaction:}
\begin{itemize}
\item \ce{RCHO + 2Cu^2+ + 5OH- ->[heat] RCOO- + Cu2O v + 3H2O}
\item With glucose: \ce{C6H12O6 + 2Cu^2+ + 5OH- -> C6H12O7 + Cu2O v + 3H2O}
\end{itemize}

\textbf{Observation:} Blue solution turns to red-brown precipitate of cuprous oxide.

\textbf{Benedict's reagent:} Similar test using copper citrate complex instead of tartrate.

\subsubsection{Iodine clock reaction}

\textbf{Purpose:} Demonstration of reaction kinetics and reaction mechanisms.

\textbf{Principle:} A sudden color change occurs after a predictable time period, demonstrating the relationship between reaction rate and concentration.

\textbf{Common version (Landolt reaction):}
\begin{itemize}
\item Reaction A (slow): \ce{H2O2 + 2I- + 2H+ -> I2 + 2H2O}
\item Reaction B (fast): \ce{I2 + 2S2O3^2- -> 2I- + S4O6^2-}
\item When thiosulfate is consumed, free iodine reacts with starch indicator
\item \ce{I2 + starch -> blue complex}
\end{itemize}

\textbf{Alternative version (Dushman reaction):}
\begin{itemize}
\item \ce{IO3- + 3HSO3- -> I- + 3SO4^2- + 3H+} (slow)
\item \ce{IO3- + 5I- + 6H+ -> 3I2 + 3H2O} (fast, when HSO$_3^-$ depleted)
\end{itemize}

\textbf{Observation:} Solution remains colorless for a fixed time, then suddenly turns deep blue.

\textbf{Variables affecting clock time:}
\begin{itemize}
\item Concentration of reactants
\item Temperature
\item Presence of catalysts
\end{itemize}

\subsubsection{Biuret test}

\textbf{Purpose:} Test for proteins and peptide bonds; detects presence of peptide linkages.

\textbf{Reagent:} Biuret reagent (copper sulfate in alkaline solution)
\begin{itemize}
\item \ce{CuSO4} in dilute \ce{NaOH} solution
\end{itemize}

\textbf{Principle:} Peptide bonds form a colored complex with Cu$^{2+}$ ions in alkaline solution.

\textbf{Reaction:} Copper ions coordinate with nitrogen atoms of peptide bonds, forming a violet-purple complex.

\textbf{Named after:} Biuret \ce{H2N-CO-NH-CO-NH2}, the simplest compound that gives this test.

\textbf{Observation:}
\begin{itemize}
\item Negative (no protein): Blue color (from Cu$^{2+}$ ions)
\item Positive (protein present): Violet to purple color
\item Intensity depends on number of peptide bonds
\end{itemize}

\textbf{Requirements:}
\begin{itemize}
\item At least two peptide bonds required for positive test
\item Single amino acids do not give positive result
\item Dipeptides give weak positive result
\item Tripeptides and proteins give strong positive result
\end{itemize}

\textbf{Application:}
\begin{itemize}
\item Qualitative test for proteins
\item Semi-quantitative determination of protein concentration
\item Used in biochemistry and food analysis
\end{itemize}

\subsubsection{Lucas test}

\textbf{Purpose:} Distinguish between primary, secondary, and tertiary alcohols.

\textbf{Reagent:} Lucas reagent (anhydrous \ce{ZnCl2} in concentrated \ce{HCl})

\textbf{Principle:} Alcohols react with HCl in presence of \ce{ZnCl2} to form alkyl chlorides (insoluble, appears as cloudiness).

\textbf{Reactions:}
\begin{itemize}
\item \ce{ROH + HCl ->[ZnCl2] RCl + H2O}
\item Rate: Tertiary > Secondary > Primary
\end{itemize}

\textbf{Observations:}
\begin{itemize}
\item Tertiary alcohol: Immediate cloudiness (turbidity)
\item Secondary alcohol: Cloudiness within 5-10 minutes
\item Primary alcohol: No reaction at room temperature
\end{itemize}

\subsubsection{Diazotization reaction}

\textbf{Purpose:} Formation of diazonium salts from primary aromatic amines.

\textbf{Reagent:} Sodium nitrite (\ce{NaNO2}) and dilute \ce{HCl} at 0-5$^\circ$C

\textbf{Reaction:}
\begin{itemize}
\item \ce{C6H5NH2 + NaNO2 + 2HCl ->[cold] C6H5N2+Cl- + NaCl + 2H2O}
\item Temperature must be kept low to prevent decomposition
\end{itemize}

\textbf{Applications:}
\begin{itemize}
\item Azo dye synthesis (coupling with phenols or aromatic amines)
\item Sandmeyer reaction (replacement of diazonium group)
\item Gattermann reaction
\end{itemize}

\subsubsection{Friedel-Crafts reactions}

\textbf{Friedel-Crafts alkylation:}
\begin{itemize}
\item \ce{C6H6 + RCl ->[AlCl3] C6H5R + HCl}
\item Introduces alkyl group onto aromatic ring
\item Catalyst: \ce{AlCl3} or \ce{FeCl3}
\item Problem: Polyalkylation and rearrangement
\end{itemize}

\textbf{Friedel-Crafts acylation:}
\begin{itemize}
\item \ce{C6H6 + RCOCl ->[AlCl3] C6H5COR + HCl}
\item Introduces acyl group onto aromatic ring
\item More controlled than alkylation (no polyacylation)
\item Forms ketones
\end{itemize}

\subsubsection{Kolbe's reaction (Kolbe-Schmitt reaction)}

\textbf{Purpose:} Synthesis of salicylic acid from phenol.

\textbf{Reaction:}
\begin{itemize}
\item \ce{C6H5OH + NaOH -> C6H5ONa + H2O}
\item \ce{C6H5ONa + CO2 ->[heat, pressure] C6H4(OH)COONa} (sodium salicylate)
\item \ce{C6H4(OH)COONa + HCl -> C6H4(OH)COOH + NaCl} (salicylic acid)
\end{itemize}

\textbf{Conditions:} High pressure (5-7 atm), 125-130$^\circ$C

\textbf{Application:} Industrial production of aspirin (acetylsalicylic acid)

\subsubsection{Cannizzaro reaction}

\textbf{Purpose:} Disproportionation of aldehydes lacking $\alpha$-hydrogen.

\textbf{Principle:} In strong base, one aldehyde molecule is oxidized to carboxylate, another is reduced to alcohol.

\textbf{Reaction:}
\begin{itemize}
\item \ce{2RCHO + NaOH ->[no alpha-H] RCOONa + RCH2OH}
\item Example: \ce{2C6H5CHO + NaOH -> C6H5COONa + C6H5CH2OH}
\item Formaldehyde: \ce{2HCHO + NaOH -> HCOONa + CH3OH}
\end{itemize}

\textbf{Requirement:} Aldehyde must lack $\alpha$-hydrogen atoms.

\subsubsection{Haloform reaction}

\textbf{Purpose:} Test for methyl ketones; produces haloform.

\textbf{Reagent:} Halogen (\ce{I2}, \ce{Br2}, or \ce{Cl2}) in alkaline solution

\textbf{Reaction:}
\begin{itemize}
\item \ce{CH3COR + 3I2 + 4NaOH -> RCOONa + CHI3 v + 3NaI + 3H2O}
\item Iodoform (\ce{CHI3}): Yellow precipitate with characteristic odor
\end{itemize}

\textbf{Positive test:}
\begin{itemize}
\item Methyl ketones: \ce{R-CO-CH3}
\item Acetaldehyde: \ce{CH3CHO}
\item Ethanol: \ce{CH3CH2OH} (oxidized to acetaldehyde first)
\end{itemize}

\textbf{Iodoform test:} Specific for compounds with \ce{CH3CO-} or \ce{CH3CH(OH)-} structure.