Chemistry IV

Complex compounds are molecular or ionic species in which a central metal ion is surrounded by one or more ligands (ions or neutral molecules) that donate electron pairs to it through coordinate (dative) bonds. The metal and its ligands form a single stable structural unit called a coordination sphere, whose identity is retained in solution and in the solid state. The number of coordinate bonds formed by the central metal ion is its coordination number.

Examples: tetramminecopper(II) sulphate, $\mathrm{[Cu(NH_{3})_{4}]SO_{4}}$; potassium ferrocyanide, $\mathrm{K_{4}[Fe(CN)_{6}]}$; tetraamminezinc(II) sulphate, $\mathrm{[Zn(NH_{3})_{4}]SO_{4}}$.

Three complex-forming reactions with NaOH

1. With copper(II) sulphate (excess NaOH): a pale blue precipitate of copper(II) hydroxide first forms, which dissolves in excess NaOH to give a deep blue soluble complex, sodium tetrahydroxocuprate(II).

$\mathrm{CuSO_{4} + 2NaOH \rightarrow Cu(OH)_{2} \downarrow + Na_{2}SO_{4}}$

$\mathrm{Cu(OH)_{2} + 2NaOH \rightarrow Na_{2}[Cu(OH)_{4}]}$

2. With zinc sulphate (excess NaOH): a white gelatinous precipitate of zinc hydroxide forms, which dissolves in excess NaOH to give the soluble complex sodium tetrahydroxozincate(II).

$\mathrm{ZnSO_{4} + 2NaOH \rightarrow Zn(OH)_{2} \downarrow + Na_{2}SO_{4}}$

$\mathrm{Zn(OH)_{2} + 2NaOH \rightarrow Na_{2}[Zn(OH)_{4}]}$

3. With aluminium sulphate (excess NaOH): a white gelatinous precipitate of aluminium hydroxide forms, which dissolves in excess NaOH to give the soluble complex sodium tetrahydroxoaluminate(III).

$\mathrm{Al_{2}(SO_{4})_{3} + 6NaOH \rightarrow 2Al(OH)_{3} \downarrow + 3Na_{2}SO_{4}}$

$\mathrm{Al(OH)_{3} + NaOH \rightarrow Na[Al(OH)_{4}]}$

In each case the amphoteric hydroxide dissolves in excess alkali, and the metal ion becomes the central atom of a hydroxo complex.

परिचय (Definition of Solvent)

विलायक (Solvent) त्यो पदार्थ हो जसले विलेय (Solute) लाई आफ्नो भित्र घोलेर घोल (Solution) बनाउँछ। विलायक सामान्यतया घोलमा सबैभन्दा बढी मात्रामा उपस्थित हुन्छ। विलायकको प्रकृति (भौतिक र रासायनिक गुणहरू) ले त्यो कुन प्रकारको विलेयलाई घोल्न सक्छ भन्ने निर्धारण गर्दछ।

विलायकहरूको वर्गीकरण (Classification of Solvents)

विलायकहरूलाई तिनीहरूको भौतिक र रासायनिक गुणहरूका आधारमा निम्न प्रकारले वर्गीकरण गर्न सकिन्छ:

क. भौतिक गुणका आधारमा वर्गीकरण (Classification based on Physical Properties)

भौतिक गुणको आधारमा विलायकहरूलाई तिनीहरूको ध्रुवीयता (Polarity) को आधारमा वर्गीकरण गरिन्छ।

१. ध्रुवीय विलायक (Polar Solvents):

• परिभाषा: यी विलायकहरूमा एक निश्चित द्विध्रुवीय आघूर्ण (Dipole Moment) हुन्छ। यसको अर्थ तिनीहरूको आणविक संरचनामा इलेक्ट्रोनको वितरण असमान हुन्छ।
• गुण: ध्रुवीय विलायकहरूले ध्रुवीय (Polar) र आयनिक (Ionic) विलेयहरूलाई राम्ररी घोल्न सक्छन्।
• उदाहरण: पानी ($\text{H}_2\text{O}$), इथेनॉल ($\text{C}_2\text{H}_5\text{OH}$), एसिटोन ($\text{CH}_3\text{COCH}_3$)।

२. अध्रुवीय विलायक (Non-polar Solvents):

• परिभाषा: यी विलायकहरूमा आणविक संरचनामा इलेक्ट्रोनको वितरण लगभग समान हुन्छ र तिनीहरूको द्विध्रुवीय आघूर्ण नगण्य हुन्छ।
• गुण: अध्रुवीय विलायकहरूले अध्रुवीय (Non-polar) विलेयहरूलाई मात्र घोल्न सक्छन्।
• उदाहरण: हेक्सेन ($\text{C}_6\text{H}_{14}$), बेंजीन ($\text{C}_6\text{H}_6$), क्लोरोफॉर्म ($\text{CHCl}_3$)।

(भौतिक गुणको आधारमा मुख्य सिद्धान्त: "Like dissolves like" - समान प्रकृतिले समानलाई घोल्छ।)

ख. रासायनिक गुणका आधारमा वर्गीकरण (Classification based on Chemical Properties)

रासायनिक गुणको आधारमा विलायकहरूलाई तिनीहरूको हाइड्रोजन बन्डिङ (Hydrogen Bonding) गर्ने क्षमताको आधारमा वर्गीकरण गरिन्छ।

१. प्रोटिक विलायक (Protic Solvents):

• परिभाषा: यी विलायकहरूमा हाइड्रोजन परमाणु (H) कुनै अत्यधिक इलेक्ट्रोन-आकर्षक परमाणु (जस्तै: O वा N) सँग जोडिएको हुन्छ। यसले गर्दा तिनीहरूले हाइड्रोजन बन्ड दान (Donate) गर्न सक्छन्।
• गुण: प्रोटिक विलायकहरूले आयनहरूलाई स्थिर गर्न र रासायनिक प्रतिक्रियाहरूमा भाग लिन सक्छन्।
• उदाहरण: पानी ($\text{H}_2\text{O}$), मेथनॉल ($\text{CH}_3\text{OH}$), इथेनॉल ($\text{C}_2\text{H}_5\text{OH}$)।

२. अप्रोटिक विलायक (Aprotic Solvents):

• परिभाषा: यी विलायकहरूमा हाइड्रोजन परमाणु कुनै अत्यधिक इलेक्ट्रोन-आकर्षक परमाणुसँग जोडिएको हुँदैन (वा यदि जोडिएको भए पनि त्यो हाइड्रोजन बन्ड दान गर्न सक्दैन)। तिनीहरूले हाइड्रोजन बन्ड ग्रहण (Accept) गर्न सक्छन् तर दान गर्न सक्दैनन्।
• गुण: यी विलायकहरूले ध्रुवीय विलेयहरूलाई घोल्न सक्छन्, तर प्रोटिक विलायकहरू जत्तिको आयन स्थिरीकरण क्षमता हुँदैन।
• उदाहरण: डाइमिथाइल सल्फोक्साइड (DMSO), डाइमिथाइलफोर्मामाइड (DMF), एसीटोन ($\text{CH}_3\text{COCH}_3$)।

Adsorption is the phenomenon in which molecules of a gas or solute (the adsorbate) accumulate and concentrate on the surface of a solid or liquid (the adsorbent), rather than penetrating into its bulk. It occurs because the surface atoms of a solid have unsatisfied residual valencies (free surface energy), so they attract and hold other molecules at the surface. Adsorption is a surface phenomenon, is generally exothermic, and is accompanied by a decrease in entropy. It differs from absorption, in which the substance is distributed uniformly throughout the bulk of the other substance.

Types: Physical adsorption (van der Waals forces, low heat of adsorption, reversible, multilayer) and chemical adsorption or chemisorption (chemical bond formation, high heat of adsorption, irreversible, monolayer).

Applications of adsorption

1. In catalysis: Finely divided metals such as Ni, Pt, Pd and Fe adsorb reactant molecules on their large surface and increase the rate of reaction — e.g. Haber process (Fe), hydrogenation of oils (Ni), contact process (Pt/V₂O₅).
2. Purification of water and decolourisation: Animal charcoal, activated charcoal and silica gel adsorb colouring matter, odours and organic impurities from water, sugar solution and air.
3. Gas masks: Activated charcoal in gas masks adsorbs poisonous gases and protects workers in mines and chemical factories.
4. Chromatography: Adsorption of solutes on adsorbents such as alumina or silica gel is the basis of separation, identification and purification of mixtures (column, paper and thin-layer chromatography).
5. Drying and dehumidification: Silica gel and activated alumina adsorb moisture from air and keep electronic equipment, instruments and medicines dry.
6. Removal of hardness of water: Zeolites (permutit) adsorb Ca²⁺ and Mg²⁺ ions from hard water by ion exchange, making it soft.
7. Adsorption indicators and precipitation: In titration, dyes such as eosin and fluorescein are adsorbed on the precipitate surface and change colour at the end point.
8. In medicine and pharmacy: Activated charcoal is used to adsorb poisons in the stomach in cases of poisoning; drugs are adsorbed on adsorbents for slow release.
9. In the sugar and petroleum industry: Bone charcoal decolourises sugar; fuller's earth adsorbs impurities from lubricating oils.
10. Vacuum production and surface science: Adsorption of gases on charcoal at low temperature helps create high vacuum; measurement of surface area of solids is done by adsorption isotherms (Freundlich and Langmuir).

Thus adsorption is of great practical importance in industry, medicine, environmental protection and laboratory work.

Galvanic cell (Voltaic cell)

A galvanic cell is an electrochemical cell in which a spontaneous redox reaction is used to convert chemical energy into electrical energy. It consists of two half-cells (two different electrodes dipped in their respective electrolyte solutions) joined by a salt bridge or a porous pot. The electrode with higher reduction potential acts as the cathode (positive terminal) and the electrode with lower reduction potential acts as the anode (negative terminal). Electrons flow from anode to cathode through the external wire. Example: Daniel cell, in which zinc in ZnSO₄ solution and copper in CuSO₄ solution are connected; the cell reaction is

$\text{Zn(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Zn}^{2+}\text{(aq)} + \text{Cu(s)}$

Electrolytic cell

An electrolytic cell is an electrochemical cell in which electrical energy from an external source is used to bring about a non-spontaneous redox reaction (electrolysis). It has two electrodes dipped in the same electrolyte (or in molten electrolyte) and no salt bridge is needed. Here the anode is connected to the positive terminal and the cathode to the negative terminal of the battery. Oxidation occurs at the anode and reduction at the cathode. Example: electrolysis of molten NaCl, where Cl⁻ ions are oxidized at the anode and Na⁺ ions are reduced at the cathode.

Redox reaction in an electrochemical cell

In both types of cell, the overall reaction is a redox reaction made of two half-reactions:

• Oxidation half-reaction (at anode): loss of electrons, e.g. Zn → Zn²⁺ + 2e⁻
• Reduction half-reaction (at cathode): gain of electrons, e.g. Cu²⁺ + 2e⁻ → Cu

The electrons released by oxidation at the anode travel through the external circuit to the cathode, where they are consumed in reduction. The number of electrons lost equals the number of electrons gained, so the two half-reactions balance. In a galvanic cell this electron flow is spontaneous and drives the current; in an electrolytic cell the same type of electron transfer is forced to occur in the opposite (non-spontaneous) direction by an external voltage. Thus, in every electrochemical cell, oxidation at the anode and reduction at the cathode occur simultaneously, and the sum of the two half-reactions gives the net cell reaction.

Why methylene hydrogen atoms are acidic in malonic ester? How would you prepare carboxylic acids from malonic ester?

Acidity of the methylene hydrogens

Malonic ester is diethyl malonate, $\mathrm{CH_2(COOC_2H_5)_2}$. The central $\mathrm{CH_2}$ lies between two ester carbonyl groups.

1. Inductive effect. Each $-\mathrm{COOC_2H_5}$ group withdraws electrons ($-I$ effect). Two such groups on one carbon polarise the $\mathrm{C-H}$ bonds strongly, so the hydrogen is easily removed as a proton.

2. Resonance stabilisation of the conjugate base. On losing $\mathrm{H^+}$ the carbanion (enolate) formed is stabilised because the negative charge is delocalised onto both carbonyl oxygens:

$\mathrm{^-CH(COOEt)_2}\;\longleftrightarrow\;\mathrm{EtO-C(O^-)=CH-COOEt}\;\longleftrightarrow\;\mathrm{EtOOC-CH=C(O^-)-OEt}$

3. Comparison. The $\mathrm{p}K_a$ of malonic ester is about 13, against about 25 for ethyl acetate (one ester group) and about 50 for an alkane. So the methylene hydrogens are acidic enough to be removed completely by sodium ethoxide.

Preparation of carboxylic acids (malonic ester synthesis)

Step 1: Enolate formation. Treat malonic ester with sodium ethoxide in ethanol:

$\mathrm{CH_2(COOEt)_2 + EtO^- \rightarrow {}^-CH(COOEt)_2 + EtOH}$

Step 2: Alkylation. The enolate attacks a primary alkyl halide $\mathrm{RX}$ by an $S_N2$ reaction:

$\mathrm{{}^-CH(COOEt)_2 + RX \rightarrow R-CH(COOEt)_2 + X^-}$

(Repeating steps 1 and 2 with a second halide $\mathrm{R'X}$ gives $\mathrm{RR'C(COOEt)_2}$.)

Step 3: Hydrolysis. Boil with aqueous alkali followed by acidification (or with dilute acid) to convert both ester groups into carboxyl groups:

$\mathrm{R-CH(COOEt)_2 + 2H_2O \rightarrow R-CH(COOH)_2 + 2EtOH}$

Step 4: Decarboxylation. On heating to about $150\,^\circ\mathrm{C}$ the substituted malonic acid loses $\mathrm{CO_2}$ (it is a $\beta$-dicarboxylic acid):

$\mathrm{R-CH(COOH)_2 \xrightarrow{\Delta} R-CH_2-COOH + CO_2}$

Overall. The product $\mathrm{RCH_2COOH}$ has two more carbons than the halide $\mathrm{RX}$ (the $\mathrm{CH_2}$ carbon and one carboxyl carbon). For example, ethyl bromide gives butanoic acid:

$\mathrm{C_2H_5Br \rightarrow C_2H_5-CH(COOEt)_2 \rightarrow C_2H_5-CH(COOH)_2 \xrightarrow{\Delta} C_3H_7COOH + CO_2}$

The method works with primary (and some secondary) alkyl halides; tertiary halides give elimination, and aryl or vinyl halides do not react.

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OR

Why toluene is more reactive than benzene? Explain with reason, whether toluene is an ortho-para directing or meta-directing towards the electrophilic substitution reactions.

Why toluene is more reactive than benzene

1. $+I$ effect. The methyl group releases electrons to the ring because the $sp^3$ carbon is less electronegative than the $sp^2$ ring carbon.

2. Hyperconjugation. The $\mathrm{C-H}$ bonds of $\mathrm{CH_3}$ overlap with the ring $\pi$-system and push electron density into it, mainly at the ortho and para positions.

3. Stabler intermediate. Electrophilic substitution proceeds through a carbocation (arenium ion, $\sigma$-complex). The methyl group stabilises this cation, which lowers the activation energy. Toluene therefore undergoes nitration, halogenation and sulphonation faster than benzene (about 25 times faster in nitration).

Directing effect of the methyl group

The methyl group is an activating, ortho-para directing group.

Reason. When the electrophile attacks the ortho or para position, one resonance structure of the arenium ion has the positive charge on the ring carbon that carries $\mathrm{CH_3}$. This is a tertiary carbocation, stabilised by the $+I$ effect and hyperconjugation of the methyl group, so it is especially stable. When attack is at the meta position, the positive charge is never on the carbon bearing $\mathrm{CH_3}$, so no such extra stabilisation occurs. The ortho and para transition states are therefore lower in energy, and substitution occurs mainly at these positions.

Product distribution. Nitration of toluene gives roughly 58% ortho, 37% para and 5% meta nitrotoluene. The ortho positions are two in number but somewhat hindered by the methyl group.

Conclusion. Toluene is more reactive than benzene because $\mathrm{CH_3}$ activates the ring by $+I$ effect and hyperconjugation, and for the same reason it directs the incoming electrophile to the ortho and para positions.

Mass Spectrometry: General Principle and Applications

General Principle

Mass spectrometry (MS) is an analytical technique used to measure the mass-to-charge ratio (m/z) of ions in the gas phase. It works on the following general principle:

1. Ionization: The sample is first vaporized and then bombarded with a beam of high-energy electrons (usually 70 eV) in an ionization chamber. This knocks out electrons from the sample molecules and produces positively charged ions (cations), e.g.: $\text{M} + e^- \rightarrow \text{M}^+ + 2e^-$

2. Acceleration: The positively charged ions are accelerated by an electric field so that all ions acquire the same kinetic energy.

3. Deflection (Separation): The accelerated ions are passed through a magnetic or electric field. The field deflects the ions; lighter ions (small m/z) are deflected more, while heavier ions (large m/z) are deflected less. Thus ions of different masses are separated.

4. Detection: The separated ions strike a detector (photographic plate or electron multiplier), producing an electrical signal. The signal is recorded as a mass spectrum — a plot of relative abundance versus m/z.

The basic relationship governing deflection is: $\frac{m}{z} = \frac{H^2 r^2}{2V}$ where H = magnetic field strength, r = radius of curvature, V = accelerating voltage.

Applications

1. Determination of molecular mass: The molecular ion peak (M⁺) gives the molecular mass of the compound.
2. Determination of molecular formula: From the M⁺ peak and isotope peaks (M+1, M+2), the molecular formula can be deduced.
3. Structural elucidation: Fragmentation patterns give information about the structure of organic compounds.
4. Detection of isotopes: Mass spectrometry identifies isotopes and their relative abundance.
5. Qualitative and quantitative analysis: Used to identify unknown compounds and to determine the amount of a substance in a mixture.
6. Impurity detection: Traces of impurities in drugs, foods and environmental samples can be detected.
7. Biochemical and medical applications: Used in proteomics, drug testing, and diagnosis of metabolic disorders.
8. Space and environmental research: Used to analyse atmospheric and extraterrestrial samples.

Conclusion: Mass spectrometry is a highly sensitive and versatile analytical tool based on the separation of ionized molecules according to their mass-to-charge ratio, and it has wide applications in chemistry, biology, medicine and environmental science.