Monday, 23 May 2011

Chemistry: Typical Organic reactions

No pictures here, but it'll appear in the doc. ver.
Also, it seems that there're several out.syb. stuffs here. including:
- the term electrophilic addition
- diol by OsO4
- alc. KOH
- pyridinium chlorochromate
1)       Alkane
-          Combustion: under complete combustion we have CnH2n+2+(3n+1)/2 O2 → nCO2+(n+1)H2O
-          Halogenation: substituting a halogen atom into a alkane CnH2n+2 + X2 → C2H2n+1X + HX under un light or heat. (uv is better) Under excess alkane the major product is C2H2n+1X where reaction take place on each molecule once only. Under excess halogen, all hydrogen attached to carbon is substituted by halogen, and becomes CnX2n+2.
2)       Alkene
-          Electrophilic addition: RCH=CHR1 + X-Y → RCHXCHYR1.
e.g. addition of hydrogen or addition of halogen in organic solvents [2].
-          Hydration by cold, concentrated H2SO4
-          When HA is added to asymmetrical alkene, Markovnikov's rule states that the major product is that the hydrogen atom is added to the carbon atom already carrying more hydrogen atoms. For example, when Br2 is added to propene, the major product is 2-bromopropane since the there're more H on the first carbon atom.
-          This is also a test for existence of double/triple bond. When bromide is added to unsaturated compounds, it changes from brown to colourless.
-          Oxidizes into diol by cold, dilute K2Cr2O7/OH- or OsO4.
3)       Haloalkane
-          Preparation by dehalogenation under alcoholic alkaline environment
-          Zaitsev's rule states that under elimination the substituent on C with less H are more likely to be eliminated. For example, when 2-chlorobutane is undergoing dehalogenation, but-2-ene is the major product instead of but-1-ene.
-          Substitution to alcohol under NaOH and reflux.
4)       Alcohol
-          There are three classes of alcohol, including primary secondary and tertiary alcohol, depending on the number of alkyl groups attached to the C atom linked with the hydroxyl group. For example, ethanol is primary; cyclopentanol is secondary while 2-methyl-propan-2-ol is tertiary.
-          Primary and secondary alcohols become haloalkane under anhydrous environment and ZnCl2.
-          Tertiary alcohols readily changes into haloalkane under concentrated HCl and room temperature.
-          HBr is prepared by H2SO4+NaBr while HI is prepared by H3PO4+NaI in laboratory.
-          React with PBr3: 3ROH + PBr3 → 3RBr + H3PO3 under reflux. PBr3 is prepared by mixing red phosphorus and bromine.
-          React with PCl5: ROH + PCl5 → RCl + POCl3 + HCl
-          React with SOCl2: ROH + SOCl2 → RCl + SO2 + HCl under reflux.
-          Dehydration on 1º alcohol by excess conc. H2SO4 and heating.
-          Dehydration on 2º alcohol by 80% H2SO4 and heating.
-          Dehydration on 3º alcohol by 20% H2SO4 and heating.
5)       Oxidation among alcohol, aldehyde and ketone
Under oxidation, 1º alcohol → aldehyde → carboxylic acid
The common oxidizing agents include acidified/alkaline potassium permanganate (KMnO4/H+ or /OH-) and acidified potassium dichromate (K2Cr2O7/H+).
For example, the oxidation of ethanol to ethanoic acid by acidified potassium permanganate.
Note that under such strong oxidizing agent, the aldehyde is not isolatable, if aldehyde is to be obtained, we can use PCC(pyridinium chlorochromate) as [O].
Under oxidation, 2º alcohol → ketone by similar oxidizing agent. No further reactions occurred.
There is no oxidation process for 3º alcohol since aldehyde can't be formed (=O) without kicking one of the alkyl group and this is impossible.
6)       Reduction among alcohol, aldehyde and ketone
-          Under reduction carboxylic acid, aldehyde or ketone reduces back to alcohol.
-          Lithium aluminium hydride (LiAlH4) must be applied in anhydrous environment since it's explosive with water, so dry ether (ethoxyethane/diethyl ether/CH3CH2OCH2CH3) as the solvent. Also acid must be put in separate step, we can write like the example:
-          NaBH4 is a weaker reducing agent which can't reduce carboxylic acid.
7)       Carboxylic acids
Condensation is the reaction in which two or more molecules react together to form a larger molecule with the elimination of a small molecule like H2O.
Esters is a good solvent and have a sweet smell so it's used as artificial flavorings.
Fischer esterfication is the reversible reaction R1COOH+R2OH→R1COOR2+H2O under conc. H2SO4.
Preparing amide:
2CH3COOH + (NH2)2CO3 2CH3COO-NH4 + CO2 + H2O or RCOOH + NH3 RCOO4NH4+ help us to prepare the ammonium carboxylate, then by heating RCOO4NH4+ RCONH2 + H2O. Excess conc. ethanoic acid is used to prevent the dissociation of the salt before it dehydrates.
8)       Esters
-          (Acidic) hydrolysis: by adding water to ester it becomes alcohol and carboxylic acid.
-          In alkaline solution, we can add NaOH to produce RCOO-Na+. Since this removes the carboxylic acid, the equilibrium goes to complete. Remove the alcohol by distillation in reflux set up. After ROH is collected we obtain RCOOH by adding excess mineral acid.
9)       Amides
-          In acidic solution: RCONH2 + H3O+ RCOOH + NH4+, e.g. CH3CONH2+HClCH3COOH+NH4Cl
-          In alkaline solution: RCONH2 + OH- RCOO- + NH3, e.g. CH3CONH2+NaOHCH3COO-Na++NH3
1)       We won't discuss the regionospecific properties here.
2)       Adding halogen in aqueous form gives different product. e.g., CH2=CH2+Br2(aq)→CH2BrCH2OH
3)       Alkene also undergoes polymerization.
4)       Reflux and related apparatus will be discussed later.
5)       The order of reactivity is always RI>RBr>RCl>>RF (R can by alkyl or hydrogen) because of the stability of halide ions. HF usually does not react.
6)       The reactivity of alcohols are given by 3º>2º>1º since the more alkyl group attached, the more stable the alkyl ion.

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