Saturday 25 December 2010

Chemistry : Introduction to carbon chemistry

Carbon chemistry
Uniqueness of carbon:
1)       Ability to catenate into a long chain, i.e., forming stable bonds with itself steadily (C-C bond), Note that Si can’t catenate since Si-Si bond is weak.
2)       Forming multiple bonds since it can process sp3, sp2 and sp hybridization
3)       Forming 4 covalent bonds.
Nomenclature and expression:
-          Condensed structural formula: omit C-C and C-H bond in the parent chain. E.g. CH3CH3.
Use bracket for branched chain, e.g. CH3CH(CH3)CH3
-          Skeletal formula: omitting C atom, C-C bond and C-H bond.
Functional group is one or a group of toms that determines most properties of the compound.
A homologous series is a series of compounds that have the same functional group; each member differs from the next one by a –CH2- unit. For example, alkane: CH4, CH3CH3
They have the same general formula, shows gradual change in physical properties and similar in chemical properties.
Gradual change in physical properties down the homologous series include: darker in colour, more viscous, less volatile, less flammable and burns with a more sootier flame.
There’re two types of hydrocarbons, aliphatic hydrocarbon is those who doesn’t contain a benzene ring while aromatic hydrocarbon has at least one benzene ring. Hydrocarbons with π-bond are called unsaturated hydrocarbons.
Functional group: (R represents H or alkyl group, X represents halogen atom)
1)       Alkane is not a functional group, has a general formula of CnH2n+2 (RH).
2)       Alkene has general formula CnH2n (R2C=CR2), it contains double bond(s).
3)       Alkyne has general formula CnH2n-2 (RC≡CR), it contains triple bond(s)
4)       Haloalkane has general formula CnH2n+1X (RX), it contains halogen atom(s).
5)       Alkanol has general formula CnH2n+1OH (RO) it contains –OH group(s).
6)       Alkanoic acid has general formula Cn-1H2n+1COOH (RCOOH)
7)       Aromatic hydrocarbon contains a benzene ring or phenyl (C6H5-) group.
Basic nomenclature: prefix – parent – unsaturated form - suffix
1)       Parent chain is the longest chain that contains the functional group, triple and double bond. From 1 to 8 carbon: meth-, eth-, prop-, but-, pent-, hex-, hept- and oct-. For example, C8H18 is called octane.
2)       Assigning number to the carbon atom on the carbon chain, functional group and π-bond should be assigned a number which is the smallest possible. For example, CH3CH2OH, C is assigned 1 but not 2.
3)       Suffix: the functional group; for example, CH3CH2COOH has suffix –oic acid. When the functional group does not has a unique position, numbers are assigned before suffix. For example there’re two butene, namely but-2-ene (CH3CH=CHCH3) and but-1-ene (CH2=CHCH2CH3), the double bond appears in the first and second C atom respectively.
4)       Prefix: the substitutes, mainly branched chains, like CH3CHClCH is called 2-chloropropane, since it has 3 C atom on parent chain (prop-), has no functional group (-ane), and the Cl atom is on the second C atoms. When there’re more than one substitutes,, we use alphabetical order to arrange them.
5)       The unsaturated form shows the existence of π-bonds. Double bond is shown by –en-, it there’re more than one double bonds, we use di-, tri- and tetra-, e.g. if there’re three double bond and 2 triple bond, then the unsaturated form is –triendiyne.
6)       The name of functional group uses –yl, like CH3- is methyl and C2H5- is ethyl-
7)       When there’re more than 1 function groups, -oic acid takes higher priority than –OH, then the –OH group will be the prefix.
8)       For cycloalkanes, the functional group is assigned as the first C atoms.
More examples:
-          CH3C(CH3)2CH2CH3 has 4 C atom on parent chain, no functional groups, and 2 methyl group on the second C atom, so it’s called 2,2-dimethylbutane
-          CH3CH=CHCl has 3 C atoms on parent chain, belongs to alkene, and has 1 chlorine atom on the first C atom, so it’s called 1-chloroprop-1-ene.
-          HOCH2CH2COOH is called 3-hydroxypropanoic acid.
-          CH3C(CH3)=CBrCOOH is called 2-bromo-3methylbut-2-enoic acid.
-          The figure shown is 1-methylhexene, since the C atom connected to methyl group and double bond is assigned as the first carbon atom.
-          Benzene is cyclohex-1,3,5-triene.
Reactions of alkane:
1)       CombustionCnH2n+2+(3n+1)/2 O2→nCO2+(n+1)H2O, upon incomplete combustion, CO and C (unconsumed hydrocarbon) is produced, and sooty flame is observed. Combustion of alkane has made alkane into the main use of fuel.
2)       Substitution (Halogenation):
-          Initiation: Under UV light, X2 → 2X∙, X∙ refers to halogen free radicals.
-          Chain reaction (propagation): RH (alkane) + X∙ → R∙ + HX and R + X2 → RX + X∙
-          Termination occurs when two free radicals collides and combine: X∙+ X∙→X2, X∙+R→RX or R∙+R’∙→R2
-          Under excess alkane, the major product is monosubstituted haloalkane and if halogen is in excess, haloalkanes with all H being substituted by halogen atoms will be the main product. For example, in excess Cl2, CH4 will eventually change into CCl4. If there’re excess CH4, then major product will be CH3Cl.
3)       Cracking: Under heating of catalyst, when long-chain hydrocarbons pass through the catalyst, it cracks into shorter alkane, alkene and alkyne. It’s called catalytic cracking. It has the significant use of producing alkenes and extra fuels.

Reactions of alkene
1)       Addition : When X2 is react with alkene in organic solvent, halogen atom are attached to the two C atom in the double bond. e.g.  
It serves as test for unsaturation: Br2 in organic solvent. At the same time, alkenes under Pt/Pd/Rh/Ni as catalyst react with H2 to give alkanes.
2)       Oxidation: CH2=CH2 + [O] + H2O HOCH2CH2OH, the alkene is oxidized into a diol. It serves as a test for unsaturation: purple cold, diluted KMnO4/H+ become colourless when added to unsaturated hydrocarbon. Other oxidizing agents include KMnO4/OH- and OsO4.
Applications – fossil fuels
Coal is formed by dead land animals in a million years ago , covered by mud and under heat, pressure and effects of bacteria. Petroleum and natural gas is formed in the similar way, but it requires sea dead animals instead of that from land.
Uses of different fraction of petroleum
1) Refinery gas – fuel (LPG)                  2) Petrol – Car fuel
3) Naphtha – raw material of town gas        4) Kerosene – domestic fuel, aircraft fuel
5) Diesel oil – fuel for heavy vehicles          6) Fuel oil – fuel for ships and power stations
7) Wax, lubricating oils – candles            8) Bitumen – Road construction, roofs
Exothermic reaction is the reaction that heat is released while heat is taken in during endothermic reaction.
Air pollutants
1)       CO and C from incomplete combustion in cars, CO is toxic.
2)       Unconsumed hydrocarbons from cars like benzene are carcinogenic.
3)       Respirable suspended particle (RSP) reduces visibility and damage respiratory system.
4)       Oxides of nitrogen (NOx) from car engines, gives acid rain (2NO2+H2OHNO2+HNO3), irritate respiratory system and forms photochemical smog with unconsumed hydrocarbon.
5)       SO2 from combustion of impure fuel, is irritating and form acid rain (SO2+H2O→H2SO3)
6)       Heavy metal oxides (mainly PbO) from impurities: toxic
1)       Catalytic convertors (Pt/Pd/Rh) engages complete combustion and convert CO, NOx into less harmful CO2 and N2.
2)       Scrubbers or desulphurization systems in the power stations: SO2 is neutralized by CaO.
3)       Electrostatic precipitators in power stations and particulate removal devices in motor vehicles remove particulates.
4)       Using lead-free, low-sulphur content fuels for cards, low nitrogen oxide burners in power stations.

I've got some hard time to select those elementary reactions since it's hard to exclude some reaction when you're taking "addition" or other kinds of reactions. For example, KMnO4/H+ further oxidizes diols, so KMnO4/OH- is a better test for unsaturation, but it's not on the book.

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