Hydrocarbons PDF

# Hydrocarbons * Hydrocarbons are composed of carbon and hydrogen. * The important fuels like Petrol, kerosene, coal gas, LPG etc. are all hydrocarbons or their mixture. **Sources:** * Petroleum and natural gas are the major sources of aliphatic hydrocarbons while coal is an important source of aromatic hydrocarbons. * The oil trapped inside the earth is known as petroleum. **PetRA**- ROCK, **OLEUM**-OIL. * The oil in the petroleum field is covered with a gaseous mixture known as natural gas. * The main constituents of the natural gas are methane, ethane, propane, and butane. ## Classification of Hydrocarbons | HYDROCARBON | Acyclic or Aliphatic (Open chain) | Carbocyclic or Cyclic | | :----------------------------------- | :-------------------------------- | :------------------------ | | | Alkanes, Alkenes, Alkynes | Alicyclic, Aromatic | | | | Cycloalkanes. cycloalkenes, cycloalkynes | ## Alkanes * **Paraffins** * General formula CnH2n+2 * sp3 hybridization * C-C bond length 1.154 A° * Chemically unreactive * Show chain, position, and optical isomerism. * Heptane has 9 isomers, Octane 18, and Decane 75. **Preparation:** 1. **Wurtz Reaction:** 2CH3CH2Br + 2Na → Dry> CH3CH2CH2CH3 + 2NaBr * Follow mainly free radical mechanism * Useful in preparing an alkane containing an even number of carbon atoms. * Stepping up reaction. **Frankland reaction:** Rx + 2oT + Rx → R-R + 2nx2 2. **From Grignard reagent CRMgX7** RMgX + HOH → RH + Ma COH)x RMgX + R’OH → RH+Ma COR’X RMGX + R’NH2 → RH+Mg [NHR’) X 3. **From unsaturated hydrocarbons:** * Sabatier-Senderens reduction * R-CH=CH2 + H2 Ni/Δ > R-CH2-CH3 * R-C = CH+H2 Ni/Δ > R-CH2-CH3 4. **From carboxylic acids:** * Decarboxylation: * CH3COONa+ + NOOH → CO2 CH4 + Na2CO3 * Sodium ethanate * Kolbes electrolytic method * 2CH3COO-Na+ + 2H2O → Sodium acetate * ↓ * Electrolysis * CH3-CH3 + 2CO2 + H2 + 2NaOH ## Physical Properties 1. **Nature**: Non-Polar due to covalent nature of C-C bond and C-H bond. * C-C bond energy = 83kj/mole and C-H bond energy = 99kj/mole * C1-C4=gasses, C6 -C17 = Colorless Odourless liquid and >C17 = Solid. 2. **Solubility:** Like dissolve like * Viz, Polar compounds dissolve in polar solvent and Non-Polar compound dissolve in a non-polar solvent 3. **Boiling Point:** Low boiling point due to non polar nature. * The molecules are held together only by Van der Waalls’ forces. * Since we known that the magnitude of van der Wablls’ forces is directly proportional to the molecular size. * Therefore, the boiling point increases with an increase in the molecular size i.e. with an increase in the number of Carbon atoms. **Noted:** The boiling points of the branched chain Alkanes are less than the straight chain isomers. * This is due to the fact that branching of the chain makes the molecule more compact and thereby decreases the surface area and consequently the magnitudes of van der Waalls’ forces also decrease. * CH3CH2CH2CH2CH3 n-pentane boiling point= 309K, * H3C-CH-CH2CH2 iso-pentane boiling point= 301K, * H3C-CH-C-CH3 neo-pentane boiling point = 282.5k * CH3 * CH3 * Carbon atoms having higher melting point alkanes having immediately next lower and immediately next higher odd number of carbon atoms. ## Chemical properties 1. **Combustion:** * C4H10 + 2O2 → CO2 + 2H2O ΔH = -217.Ok calmole. 2. **Oxidation:** * CH4 + O2 → 573K 2CH3OH * CH4 + O2 Mo2O3 > HCHO + H2O Methanal 3. **Substitution:** * Halogenation: * CH4 + Cl2 UV > CH3Cl + HCl * CH3Cl > CH2Cl2 UV > CHCl3 UV > CCl4 * Noted: Iodination is a reversible reaction. So it is carried out by heating alkane in the presence of some oxidizing agent like iodic acid (HIO3) or nitric acid (HNO3) or mercuric oxide (HgO) which oxidizes HI formed during the reaction. * CH4 + I2 Heat > CH3I + HI * 5HI + HIO3 → 3H2O + 3I2 * 2HI+ 2HNO3 → 2H2O + I2 + 2NO2 * Noted: Fluorination of alkane takes place explosionally resulting even in the rupture of C-C bond in higher alkanes * **Features of Halogenations:** * The reactivity of Halogens: F2 > Cl2 > Br2 * The rate of replacement of Hydrogen of alkanes is: 3°>2°>1° * CH3CH2CH2CH3 n-Butane hv > CH3CH2CH2CH2Cl + CH3CHF12CHCH3 + Cl * **Mechanism:** Halogenations reaction take place by Free radical mechanism. * The reaction proceeds in the following Steps: * Chain initiation step: CI-CI hv > 2CI* * Chain Propagation step: CH4 + CI* → CH3 + HCl * CH3 + Cl2 → CH3Cl + CI* * Chain Termination Step: Cl+ Cl → Cl2 * CH3 + CH3 → CH3-CH3 * CH3 + CI* → CH3Cl * Nitration: * The reaction takes places by free radicals mechanism at high temp [450°C] * At high temp C-C bond is also broken so that mixture of nitroalkanes is obtained. * CH3CH2CH3 450°C CH3CH2CH2NO2 + CH3 + CH3CH2 Conc.HNO3 NO2 * 25% 40.10 10.1 * The reaction occurs as: * HO-NO2 450°C Homolytic fission HO + NO2 * RH + OH → R* + HOH * R* + °N02 → RNO2 * Sulphonation: Replacement of a hydrogen atom of alkane by -503H group. * CH3 * CHCH3 O/Δ CH3-C-CH3 * CH3 * isobutane SO3H * tert butyl sulphonic acid * The reaction occurs as: * HO-SO3 450°C Homolytic fission HO* + SO3H * RH + °OH → R° + HOH * R* + °SO2H → R-SO2H * H3C(CH2)3CH3 AICI3/HCI H3CCHCH2CH3 * n-pentane CH3 * 2-Methyl butane * Aromatization: * H3C(CH2)4CH3 Cr2O3 > Benzene * Hexane 10-20atm 773°C * This method is also called denydrogenation or hydroforming. * Similarly, heptane gives toluene, n-Octane give o-xylene and 2, methyl heptane give m-xylene. * Thermal decomposition or Pyrolysis or cracking or Fragmentation: * When higher alkanes are heated at high temp [about 700-800K] in the presence of alumina or silica Catalysts, the alkanes break down to lower alkanes and alkenes * CH3-CH2-CH3 → CH3-CH-CH2+CH3-CH3+ C2H4+CHy * Action of stem: Catalyst: nickel, alumina Al2O3 * CH4+H2O(steam) 1000°C > CO + 3H2 * This reaction is used for the industrial preparation of hydrogen from natural gas. * Isomerisation: * CH3[CH2]4 CH3 Anhy.AICl3/HCI CH3CH-[CH2]2-CH3 + CH3CH2=CH-CH2-CH3 * n-Hexane CH3 * 2-methylpentane CH3 * 3-methylpentane * Conformational isomerism: * The different molecular arrangements arising as a result of rotation around carbon-carbon bonds are called conformational isomers or rotational isomers and the phenomenon is called conformational isomerism. * Numerous possible arrangements Of ethane are possible. * Two extreme conformations are known. These are eclipsed conformation and staggered conformation. **SAWHORSE REPRESENTATION:**- * H * H-C-H * H * H-C-H * H * H * H-C-H * H * H-C-H * H * H **NEWMAN PROJECTION:** * H * H-C-H * H * H * H-C-H * H * H * H-C-H * H * H **STAGGERED → SREW ← ECLIPSEP** # ALKENES * Unsaturated hydrocarbons which have double bonds * Genral molecular formula CnH2n. * C-C bond hybridization 1.34 A°. * sp2 hybridization * When we treated Alkene with Chlorine, oily products are obtained. So Alkenes are also known as olefins. [Greek oleficant meaning oil forming]. * Show chain, positional, and geometrical isomerism. **Preparation** 1. **From Alkynes:** Alkynes on partial reduction with partially deactivated palladised charcoal known as Lindlar’s & catalyst give alkynes. * CH=CH+H2 Pd/c > CH2=CH2. * Ethyne Ethene 2. **From Haloalkanes:** dehydrohalogenation. * CE2 or 1,2 - elimination or Bit elimination] * H H Alc. KOH → CH2=CH2 #Br+H2O * CH2-CH2 * Br * Mecl * Predominant formation of a substituted alkene is formed according to Saytzeff’s rule * CH3-CH2-CH2-CH3 Alc KOH → CH3-CH=CH-CH3+CH3-CH2-CH=CH2 * Br major 3. **From Dihaloalkanes:** dehalogenation * H H * H-C-C-H * Br Br * Zn/HOAC → H C=C-H + ZnBr2 * H * Nal → H C=C-H + I2+2NaBr * H 4. **From Alcohols:** Dehydration [El-elimination] * CH3CH2CH2OH Conc.H2SO4 → 160° CH3CH=CH2 + H2O * CH3CH2CH2CH2OH Al2O3 → 300° CH3CH2CH=CH2 * **Mechanism** * CH3-CH2-CH2-OH H+ → CH3-CH2-CH2-OH+ →-H2O CH3-CH2-CH2 * CH3-CH2-CH2 * CH3-CH=CH2 * * H * CH3-CH2 * CH2 * H+ → CH3-C=CH-CH3 * CH3 * loss of H2O * from1,2 * Position * [major] * H3C-C-CH2-CH3 * CH3 * loss of H2O * from 1,3 * Position * → CH3-C-CH2-CH3 * CH2 * [minor] ## Chemical Properties * **Addition Reaction:** Alkene Show electrophilic addition reaction. * Addition of Hydrogen: * RCH = CH2 H2/Ni > RCH2CH3 * Addition of Halogens: * CH2=CH2 + Br2 CCl4 → CH2-CH2 * Br Br * CH2=CH2+Br2 H2O → Br-CH2-CH2-OH + HBr * (Brown colour) * Addition of hydrogen halides: * Addition reaction of HBr to symmetrical alkenes * CH2=CH2+H-Br ─>CH3-CH2-Br * Addition reaction of HBr to unsymmetrical alkenes * takes place according to Markovnikov Rule * **Markovnikov rule:** negative part of the addendum [adding molecule] get attached to that carbon atom which possesses lesser number of hydrogen atoms * e.g. * CH3 CH3 * C=CH2 + HBr → CH3-C-CH3 (major Product) * CH3 Br * CH3 * C-CH3 + HBr → CH3-CH-CH2Br (minor Product) * CH3 Br * **Peroxide effect or kharasch (Anti Markownikoff’s addi): ** * In 1933 Kharasch and Mayo observed that when HBr is added to an unsymmetrical double * CH3-CH=CH2 Peroxide → CH3-CH2-CH2Br * Propyl bromide * Homolysis * C6H5-C-O-O-C-C6H5 → 2C6H5-C-O * Benzoyl peroxide * i) C6H5 + H-Br → C6H6 + Br ii) Homolysis * CH3-CH=CH2 + Br → CH3-CH-CH2 * Br * Cless stable primary CH3-CH-CH2-Br * Br * (more stable secondary free radical] * iv) CH3-CH-CH2Br + HBr Homolysi → CH3-CH2-CH2Br+Br * Br * [major product] * Noted: Peroxide effect is applicable only to HBr and not to HF,HCl and HI. Addition of HF, HCl, or HI takes place according to Markovnikov’s rule even in the presence of peroxide. * Addition of water (Hydration) :- Acid catalyzed addition of water * CH3-CH=CH2+H2O H2SO4/H2O → CH3-CH-CH2 * 65-70* * CH3 * C=CH2 + H2O/H → CH3-C-CH2CH3 * H2SO4 * CH3 OH * CH3CH → CH3CH2CH2CH3 + CH3CH2CH * [major] * Cminor- # Alkynes **Chemical Properties:** * Addition Reaction:- Alkyne show electrophilic addition reaction. * Addition of Hydrogen: - Hydrogenation * CH3C = CH+2H2 Ni> CH3CH2CH3 * Propyne * Noted: It may be noted that the hydrogention Can be controlled at the alkene Stage Only. This is possible by using a Lindlar’s Catalysts or Sodium in liquid NH3 at 200k temp…. * Noted: It may be again noted that Catalytic reduction of alkynes in the presence of Lindlar’s Catalyst gives cis-alkenes while in the presence of sodium in liquid NH3 (Birch reduction gives trans-alkenes * CH3C = CCH3 H2/ lindlar → H3C C = CH3 * Catalyst H H * CH3C = CC1+3 Na/NH3(liq.) → H3C C=C-H * CH3 * CH3CH → CH3CH2CH2CH3+ CH3CH2CH * [major] * Cminor- * Polymerisation: * Linear polymerisation:- Of ethyne gives polyacetylene or polyethyne which is a high molecular weight polyene containing repeating units of [CCH=CH-CH=CH] and can be representedas. * -[CH=CH-CH=CH]n- * Cyclic polymerization-results in the formatio Of aromatic compomund. * CH * CH * CH * CH * CH * Redhot * iron tune> * 873°C * or * Addition of Halogens: * HC≡CH 2Br2 → Br Br * Br Br * Addition of hydrogen halides: * HC≡CH + 2HBr ─> CH3CHBr2 * Addition of water (Hydration]: - Acid catalyzed OH addition of water * HC≡CH + H2O ---HgSO4-----> CH2=CHOH (unstable) * H2SO4 → CH3-C-CH2CH3 * H20/H CH3CH * CH3C = CCH3 H9SO4 → CH2=CHOH * H20/H unstable → CH3-C-CH2CH3 * CH3CH * CmajorJ * Acidity of Alkynes :- Terminal alkynes are acidic in nature * HC≡CH + Na -> HC = CNO⁺ + 1/2H2 * CH3-C=C-H * ↓ * Monosodium * ethynide * + Na+NH2 * CH3-C = C¯Na⁺ + NH3 * Sodium propynide * Alkanes, alkenes and alkynes follow the Following trend in their acidic behaviour: * 1] HC = CH>H2C =CH2 >CH3-CH3 * ii] HC≡CH >CH3-CE2CH >>CH3-C≡C-CH3 * **AROMATIC HYDROCARBON** * Aromatic compounds containing a benzene ring are known as benzenoids and those not containing a benzene ring are known as non-benzenoids. * Structure of Benzene - kekule Structure * H * C * H * C-H * / * H * C * H * or * Resonance and Stability of benzene - Benzene is a hybrid of various resonating Structures. * The orbital overlapping picture benzene:- * All the six carbon atom in benzene are sp2 hybridized and these hybrid orbitals From sigma bonds * The unhybridised p orbital of carbon atom are close enough to form a r bond by lateral overlap… * The six r electrons are thus delocalised and can move freely about the six carbon nuclei. * The delocalised r electron cloud is attracted more strongly by the nuclei of the carbon Atoms than the electron cloud localized betn two carbon atoms. Therefore, présence of delocalised r electrons in benzene makes it more stable. * Aromaticity:- The compounds that follow the follow feactures off are to be considered aromatic. * i) Planarity * ii) Complete delocalisation of the & electrons in the ring * iii) Preasence of [4n+2]m electrons in the ring where n is an integer (n=0,1,2.....]. This is often refferred to as Hückel Rule * Preparation of Benzene: * i. Cyclic polymerisation of ethyne: * ii Decarboxylation of aromatic acids * Coona + NaOH → Ca O + Na2CO3 * c] Removal of proton from the Carbocation intermediate * Nitration * Conc. HNO3+Conc.H2SO4 → NO2 + H2O * Nitrobenzenel * CI * Halogenation +C12 → CI * Anhyd. AICl3 + HCl * Chloro * benzene * lii Reduction of phenol:- Phenol is reduced to benzene by passing vapours over heated zinc dust * OH * UZ + * → * Physical properties: * 1. Aromatic hydrocarbons are non-polar molecules and are usually Colourless liquids or Solids with a characteristic aroma. * 2. Aromatic hydrocarbons are immiscible with water but are readily miscible with Organic. Solvents. * 3. They burn with sooty flame. * Chemical properties * Arenes are characterised by electrophilic benzene ontreatment with excess of chlorine or * CI * Anhyd AlCl3 → CI + 6C12 darkicold → CI + 6HCI * CI CI CI CI CI CI * Hexachlorobenzene * [CGC] * Addition reactions of benzene- * 1 * II * III * IV * + 3H2 Ni/Δ → * Cyclohexane * CI * CI * CI *+ 3C12 → υν * 500k * CI * CI * CI * Benzene hexachloride. * [BHC] * 2. Meta directing group and deactivating :.- * NO2-CN, CHO, COB, -COOH, -COOR, -S03H, etc. * 3. Ortho and para directing groups and deactivating. * Halogens because of their strong -I effect, overall electron density on benzene ring decreases. However, due to resonance the electron density on o-and p-positions is greater than that at the m-position. Hence, they are also 0-and p-directing groups * CARCINOGENICITY AND TOXICITY- * Benzene and polynuclear hydrocarbons containing more than two benzene rings fuse together are toxic and said to posses cancer-producing [carcinogenic] property * Directive influence of a functional group in monosubstituted benzene:- * 1. Ortho and para directing group and activating- * -OH, -NH2-NHR, - NHCOCH3, -OCH3, -CH3, C2H5 * COH * +0-H * 40-H * -H * HO * I * II * III * IV * V

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