Alkane (2) PDF

## Organic Chemistry ### Hydrocarbons **Q. What are hydrocarbons?** **Ans.** Hydrocarbons are organic compounds containing hydrogen and carbon atoms only. **Q. Classify hydrocarbons.** **Ans.** **Hydrocarbons** - **Saturated** - (contains C-C) - Eg alkanes - **Unsaturated** - (contai...

## Organic Chemistry ### Hydrocarbons **Q. What are hydrocarbons?** **Ans.** Hydrocarbons are organic compounds containing hydrogen and carbon atoms only. **Q. Classify hydrocarbons.** **Ans.** **Hydrocarbons** - **Saturated** - (contains C-C) - Eg alkanes - **Unsaturated** - (contains C=C) or C≡C - Eg alkene - Eg alkyne **Q. What are alkanes?** **Ans.** Alkanes are saturated hydrocarbons. They form a homologous series. Their members have: - same general formula (CnH2n+2) - similar chemical properties - different physical properties - adjacent members differ by -CH2 **Alkanes can be represented by different types of formula. They are:** - Molecular formula - Structural formula - Displayed formula - Skeletal formula ### Molecular Formula It shows us the actual numbers of each type of atom in a molecule. Eg C3H6 is the molecular formula of propene. ### Structural formula It tells us about the atoms bonded to each carbon atom in the molecule. Eg H H-C=C-C-H H HH CH2=CHCH3 is the structural formula of propene ### Displayed formula It shows us all the atoms and all the bonds within a molecule. Eg H H-C=C-C-H H H is the displayed formula of preopene ### Skeletal formula A simplified version of the displayed formula is called the skeletal formula. It has all the symbols for carbon and hydrogen atoms removed, as well as the carbon to hydrogen bonds. The carbon to carbon bonds are left in place. Eg "A" is the skeletal formula for propene. All the other atoms that are not carbon or hydrogen, and their bonds are included in the skeletal formula. Eg "OH" is the skeletal formula of butan-2-ol. **Q. What is the main source of alkanes?** **Ans.** Crude oil is the source of alkanes It is a mixture of hydrocarbons – alkanes, cycloalkanes, and aromatic compounds. **Hydrocarbons are obtained from crude oil by fractional distillation.** When heated, the fraction having the least boiling point is collected from the top. This fraction has hydrocarbons which are volatile and have low RMM (short chain alkanes). The fraction having the highest boiling point is collected from the bottom. This fraction has hydrocarbons having high RMM. ### Cycloalkanes These are saturated hydrocarbons. They are made up of carbon and hydrogen only and contain C-C. They contain a ring having three or more carbon atoms. Their general formula is CnH2n. Thus, they are isomers of alkene. C3H6 Cyclopropane H H H C H C-H H H H C4H8 Cyclobutane H H H C---C H H C H H H H C5H10 Cyclopentane H H H C H C-H H C H C-H H H C6H12 Cyclohexane H H H C H C-H H C H C-H H H ### Aromatic hydrocarbons They are also known as arenes. They contain "benzene" ring: H-C= H C C-H H H C C=C-H H C C-H H (C6H6) Fig: Benzene **Q. Uses of hydrocarbons** **Ans.** 1. We get different fuels from hydrocarbon Fractional distillation of crude oil results in the formation of petrol, diesel, kerosene - Petrol → used as fuel in cars - Diesel → used as fuel in truck/lorries - Kerosene → used as fuel in aircrafts 2. Hydrocarbons such as naphtha can be used as a feedstock in petrochemical industries. **Q. Physical properties of alkane** **Ans.** The physical properties are- 1. Their physical state changes as we go down the series, from gas to liquid to solid. - C1-C4 (gas) - C5-C16 (liquid) - C17 onwards (waxy solid). 2. Density increases as we go down the series. 3. Melting and boiling point increase as we go down the series. This is because down the series, no. of carbon atoms increase, Vander Waal's force increases, more energy is needed to overcome the attraction. 4. Viscosity increases down the series. 5. Flammability decreases down the series, due to increased no. of carbon atoms. 6. They are non-polar. Thus they don't dissolve in water. **Q. How can alkanes be produced?** **Ans.** Alkanes can be produced by- - Hydrogenation of alkene - Cracking of a longer chain alkane ### Hydrogenation of Alkene Alkene + H2 → Alkane [ADDITION REACTION] H H H-C=C-H + H-H → H-C-C-H H H ethene hydrogen ethane ### Cracking of longer chain alkane Long chain alkane → Short chain alkane + alkenes C10H22 → C4H10 + C3H6 + C3H6 decane butane propene propene C10 H22 → C3H8 + C4H8 + C3H6 decane butane propane propene **Q. Chemical properties of alkane** **Ans.** Alkanes are generally unreactive. This is because they are made up of carbon and hydrogen only and that there is very small electronegativity difference between these atoms. Thus alkanes are non-polar. There is no partial positive charge (δ+) on any atom to attract nucleophiles. There is no partial negative charge (δ-) or areas of high electron density to attract electrophiles. Alkanes also contain strong sigma bonds in C-C and C-H which are difficult to break. ### Alkane Reactions **Eg - 1** a) H H H-C-C-H H H (C2H6, ethane) - Ethane is made up of carbon and hydrogen only. - There is little or no electronegativity difference between atoms. Thus nucleophilic attack is prohibited. b) H H H-C-C-H H Cl (C2H5Cl, chloroethane) - In chloroethane, chlorine has higher affinity for electrons being more electronegative than carbon. - Thus, it pulls the electron cloud towards itself. - Chlorine gains a partial negative charge. - Carbon gains a partial positive charge. - Nucleophile attacks carbon. **Eg - 2** a) H H H-C-C-H H H (C2H6, ethane) - Ethane is made up of carbon and hydrogen only. - There is little or no electronegativity difference between the atoms. Thus electrophilic attack is prohibited. b) H H H-C=C-H H H (C2H4, ethene) - Ethene contains C=C. - There is a high electron density around C=C, which is susceptible to electrophilic attack. **HOWEVER, alkanes undergo two types of reaction: combustion and free radical substitution.** ### Combustion (Reaction with oxygen) #### Complete - sufficient supply of O2 - produces CO2 and H2O - releases more energy - relatively less harmful #### Incomplete - insufficient supply of O2 - produces CO and H2O - releases less energy - relatively more harmful **Eg of complete combustion** C5H12 + 8O2 → 5CO2 + 6H2O pentane + oxygen → carbon dioxide + steam **Eg of incomplete combustion** C5H12 + 11/2O2 → 5CO + 6H2O pentane + oxygen → carbon monoxide + steam **Q. Why is incomplete combustion harmful?** **Ans.** Incomplete combustion releases carbon monoxide which is toxic. When inhaled, CO combines with haemoglobin in blood and forms carboxyhaemoglobin. This prevents oxygen transport in the body. Thus, a person dies of internal suffocation. ### Free Radical Substitution - It is the reaction between alkanes and halogens in presence of UV light. - Alkane + Halogen [UV] → Halogenalkane + hydrogen halide - This reaction is a chain reaction. It keeps going on until all the hydrogen atoms of the alkane are replaced by halogen. - Observation during the reaction: - Misly -fume of hydrogen halide - The reaction occurs in three steps: - Initiation - Propagation - Termination - Eg CH4 + Cl2 [UV] → CH3Cl + HCl #### Initiation UV rays from the sun is the source of energy. This energy is used to break the bond in chlorine molecule (Cl2) to form chlorine radical (Cl). As the Cl-Cl breaks, homolytically, each chlorine atom takes one electron. Thus two chlorine radicals are formed. Radicals contain unpaired electrons and hence are very reactive. After initiation of the reaction, the reaction mixture contains: - lots of methane molecules (CH4) - lots of chlorine molecules (Cl2) - a few chlorine radicals (Cl.) In the reaction mixture, there would be some productive collision and some unproductive collision. **Eg of unproductive collision** Cl. + Cl. → Cl-Cl Two chlorine radicals may collide with one another and result in the formation of chlorine molecule. There are a few Cl. in the reaction mixture. Thus this collision is least likely to occur. A chlorine radical (Cl.) could also collide with a chlorine molecule. Cl. + Cl-Cl → Cl-Cl + Cl. If this happens, no new product is formed in the reaction mixture. ### Propagation Productive collision helps the reaction to propagate, ie to grow in a kind of chain reaction. During propagation, a chlorine radical will attack a methane molecule. H H H-C-H + Cl. → H-C. + HCl H H C-H bond in CH4 breaks homolytically to form a methyl radical (CH3). The methyl radical formed attacks a chlorine molecule forming chloromethane and a chlorine radical. H H-C. + Cl-Cl → H-C-Cl + Cl. H H The chlorine radical formed attacks chloromethane. H H H-C-Cl + Cl. → H-C-Cl + Cl. H H CH2Cl. reacts with chlorine molecule H H-C-Cl + Cl-Cl → H-C-Cl + Cl. H Cl This goes on until CCl4 forms. Therefore, after propagation, we get a mixture of CH3Cl, CH2Cl2, CHCl3 and CCl4. ### Termination Whenever two free radicals collide with one another, they react. At a point, there would be no free radicals in the reaction mixture. That's when the chain reaction stops. Cl. + Cl. → Cl-Cl CH3. + CH3. → CH3-CH3 CH3. + Cl. → CH3-Cl **OVERALL EQUATION** CH4 + Cl2 → CH3Cl + HCl CH3Cl + Cl2 → CH2Cl2 + HCl CH2Cl2 + Cl2 → CHCl3 + HCl CHCl3 + Cl2 → CCl4 + HCl CH4 + 4Cl2 → CCl4 + 4HCl ### Problems with hydrocarbons **Ans.** - Hydrocarbons when burnt in limited supply of oxygen releases CO which is toxic. - Hydrocarbons when burnt in sufficient supply of oxygen releases CO2. It is a greenhouse gas, and thereby traps heat from sun causing global warming. Increased temperature of earth causes polar ice caps to melt. This causes sea-level to rise resulting in flooding. - CO2 is an acidic gas. It dissolves in rain water to form carbonic acid (H2CO3) which falls as acid rain. Acid rain destroys limestone containing buildings, and also affects soil pH and pH of water that hampers aquatic life. - In car engines, oxidation of nitrogen takes place at high temp. N2(g) + O2(g) = 2NO(g) 2NO(g) + O2(g) = 2NO2(g) These NO and NO2 are released in car's exhaust fumes. - Cars also release unburnt hydrocarbons (volatile organic compounds/ VOC) into the air. There are carcinogens (cause cancer) and also can form PAN, a contributor to photochemical smog. ### Catalytic Converter In car engine, the following reaction occurs: - N2 (g) + O2 (g) → 2NO (g) [endo; high temperature needed; obtained from burning of fuels] NO gas enters the car exhaust system. - Catalytic converter allows NO and CO to react in presence of Pt-Rh catalyst. - NO is reduced to N2 gas. - CO is oxidized to CO2 gas. - A redox reaction occurs. - NO (g) + CO (g) →1/2 N2 (g) + CO2 (g) - Harmful gases are converted to less harmful gases.