Chemistry - an illustrated guide to science (794128), страница 29
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Both ofthese are gases.● The five carbon alkane, pentane( C5H12), is a liquid.● The 34 carbon compound, butadecaneis a solid.●AlkenesCHSolidH HHCAll hom ologou s series of compoundsshow a gradation of physicalproperties as the carbon chain lengthincreases.HHSolidH© Diagram Visual Information Ltd.ChainlengthC2 ,CHEMISTRY OF CARBON168CHEMISTRY OF CARBONKey wordsalcoholaldehydealkenecarboxylic acidesterfunctional grouphomologousseriesketonepolymerFunctional groups andpropertiesClass ofcompoundHFunctional groups andpropertiesAll members of an hom ologou s seriesof compounds has the samefu n ction al grou p.
Because thefunctional group determines most ofthe chemistry of a compound,members of a particular homologousseries will have similar chemicalreactions.● Alken es are unsaturated compoundsbecause they all contain acarbon–carbon double bond thatmakes them very reactive.
Typically,they will undergo addition reactionswith hydrogen, halogens, and water.They also form a variety of polym ers.● Alcohols with a small relativemolecular mass are flammable liquidsand readily dissolve in water. Primaryalcohols are readily oxidized: first toaldehydes and then to carboxylicacids. Secondary alcohols are oxidizedto keton es:[ O][ O]R-CH2 -OH ➞ R-CHO ➞R-COOHprimary alcohol ➞ aldehyde ➞carboxylic acidCarboxylic acids are weak acids sincethey only partially ionize. They havesimilar reactions to fully ionizedmineral acids but they react with lessvigor.
Sodium salts of carboxylic acidsare ionic compounds. Those withshort carbon chains are readily solublein water.● Esters are volatile liquids or lowmelting solids. They are usuallyinsoluble in water but soluble inethanol and diethyl ether. Esters havesweet fruity smells and are used inperfumes, flavorings, and essences.© Diagram Visual Information Ltd.CCCHEtheneAlcoholC OHH HHCarboxylicacidHHH C C OEthanoleCHH HHHH C CCHO HO HEthanoic acidOH CEsterOHCO C HMethyl methanoateClass ofcompoundO CHTypical chemical propertyBr BrAlkeneCC+Br 2C CH HDecolorizes bromine water[ O]R-CHOH-R ➞ R-CO-Rsecondary alcohol ➞ ketone●HAlkene●FunctionalgroupExampleH HAlcoholH C C OHH HHconcH2SO4HCCHHDecolorizes bromine waterCarboxylicacid2CH3COOH + Na2CO32CH3COO–Na+ + CO2 + H2OReacts with sodium carbonate solutionEsterHCOOH3 + NaOHCan be hydrolized by alkaliHCOO–Na+ + CO3OH169Reaction summary:alkanes and alkenesCHEMISTRY OF CARBONKey wordsadditionpolymerizationalkanealkenesolventAlkanesC3H8 + 5O23CO2 + 4H2OSubstitutionCH4 + Cl 2CH3Cl + HClCrackingC8 H18H2C = CH2 + C6 H14AlkenesHydrogenationH2C = CH2 + H2H3C – CH3SubstitutionH2C = CH2 + Br 2H2C – CH2BrBrGeneral reaction alkene to alkanenCH2 = CH2(CH2 – CH2) n–Reaction of alkanes andalkenesBoth alkan es and alken es burn readilyin a good supply of air to producecarbon dioxide and water.● Crude oil is a complex mixture ofalkanes, which are separated intofractions (components) on the basis ofboiling point during the refiningprocess.
Some of these fractionsprovide gasoline, diesel, aviation fuel,and fuel oil.● The quality of gasoline (how smoothlyit burns) in indicated by its octanenumber, which ranges from 0–100: thehigher the octane number thesmoother burning the gasoline. Theoctane number is the percentage byvolume of 2,2,4-trimethylpentane (alsoknown as iso-octane) in a mixture of2,2,4-trimethylpentane and heptane,which has the same knockingcharacteristics as the gasoline beingtested.● Historically, tetraethyllead(IV)Pb(C2H5) 4 was added to gasoline as ananti-knock additive to make it burnmore smoothly.
A growing knowledgeof the poisonous nature of lead hasresulted in the development of leadfree fuels in which other anti-knockadditives, such as MTBE (methyltertbutyl ether), are used.● Crude oil contains no alkenes, butthey are produced in cracking andother refining processes. Alkenes areimportant feedstock for additionpolym erization but are also used ingasoline blending, making plasticizers,and as solven ts.● Much of the chemistry of the alkenesis the result of the reactive nature ofthe carbon–carbon double bond.Alkenes undergo addition reactionswith a variety of substances.●© Diagram Visual Information Ltd.Combustion170CHEMISTRY OF CARBONKey wordsalcoholaldehydecarboxylic acidesterethanoletheneoxidizing agentReaction summary:alcohols and acidsAlcoholsPreparation in industry1 AlcoholsThe majority of the world’s annualproduction of ethan ol is made by thecatalytic hydration of ethen e.A mixture of ethene and steam at300°C and 70 atmospheres is passedover a phosphoric acid catalyst.● Ethanol is also made industrially bythe fermentation of carbohydrates.● It can also be prepared in thelaboratory using concentrated sulfuricacid and heat.● Ethanol burns readily in air.
In somecountries it is used as a blending agentin motor fuels.● Alcohols can be oxidized to carboxylicacids by heating with a suitableoxidizin g agen t such as acidifiedpotassium dichromate. The oxidationinvolves two stages and goes via agroup of compounds called aldehydes.Under suitable conditions, the ethanalcan be removed from the reactionmixture before it is further oxidized toethanoic acid.●2 AcidsSalts of short-chain carboxylic acids,like sodium ethanoate, are ioniccompounds and are soluble in water.● Ethanoic acid and ethanol react in thepresence of a concentrated sulfuricacid catalyst to form the ester ethylethanoate.
This reaction is reversed byheating ethyl ethanoate with an alkalisuch as sodium hydroxide solution.The sodium salt formed, sodiumethanoate, can be neutralized by dilutemineral acid to regenerate ethanoicacid.© Diagram Visual Information Ltd.●esterificationCH2 = CH2 + H2OH3PO4 at 30 0 °C+70 AtmospheresCH3CH2OHFermentationC12H22O11 + H2Oenzymecatalyst2C6 H12O6enzymecatalyst4CH3CH2OH + 4CO2Preparation in the laboratoryCH2 = CH2 + H2Oconc H2SO4CH3CH2OH+ heatOxidation by burningCH3CH2OH + 3O22CO2 + 3H2OOxidation by oxidizing agentCH3CH2OHK2CR2O7 + dil H2SO4CH3 + COOHReaction to produce an esterCH3CH2OH + CH3CO2HconcH2 SO4H2O + CH3CO2CH2CH3Organic acidsReaction giving ionic saltCH3CO2 + NaOHCH3COO–Na+ + H2OReaction giving covalent esterCH3CO2 + CH3CH2OHCH3COOCH2CH3 + H2Oethanoic acid + ethanol ➞ethyl ethanoatehydrolysisethyl ethanoate ➞ethanoic acid + ethanolReaction giving hydrolysis of an esterCH3COOCH2CH3 + NaOHCH3COO–Na+ + CH3CH2OH171Optical isomerismCHEMISTRY OF CARBONKey words1 Chiral moleculechiralenantiomeroptical isomerismracemateWasymmetric carbon atomCOptical isomerism●XZYOptical isom erism is a form ofisomerism in which two isomers arethe same in every way except that theyare mirror images that cannot besuperimposed on each other.1 Chiral molecule●2 EnantionersmirrorClClWhen four different groups areattached to a carbon atom, theresulting molecule has no symmetry.The molecule is said to be chiral, andthe carbon atom at the center isdescribed as asymmetric.2 EnantiomersHBrCH3HH3 CBrrotate3 Optical activityClthese are notthe same moleculeHChiral molecules are said to beoptically active since they rotate theplane of polarized light.
If polarizedlight is passed through a solutioncontaining only one of theenantiomers, the plane of the light willbe rotated either to the right (dextrorotatory) or to the left (laevo-rotatory).A similar solution containing only theother enantiomer will rotate the planeof the light by the same amount in theopposite direction.● A solution containing equal amountsof the enantiomers is called a racemicmixture or racem ate. It is opticallyinactive since the two effects canceleach other out.●CBr1-bromo-1-chloroethane is a chiralmolecule.
It exists in two forms, calleden an tiom ers, that differ only in theway that the bonds are arranged inspace.● The enatiomers of a chiral moleculeare mirror images of each other andcannot be superimposed on eachother.●CCH33 Optical activityPolarizeddextro-rotatory or (+) rotatorylightlaevo-rotatory or (–) rotatory© Diagram Visual Information Ltd.C172CHEMISTRY OF CARBONKey wordsamineamino acidcarboxylic acidfunctional groupoptical isomerismproteinzwitterionAmino acids and proteins1 Amino acidsCH2H2N—CH—COOHH2N—CH—COOHalanine1 Amino acidsAm in o acids are compounds thatcontain both am in e ( -NH2) and acarboxylic acid ( -COOH) fu n ction algrou ps.● Amino acids are generally crystallinesolids that decompose on melting.They are soluble in water andinsoluble in organic solvents such asethanol.Hglycine●NH2COOHC OCH2CH2H2N—CH—COOHH2N—CH—COOHaspartic acid2 Alanine●Like most !-amino acids, alaninecontains an asymmetric carbon atomand exhibits optical isom erism .
Thereare two forms of alanine; L-alanine andD-alanine (L= laevo-[left] rotatory;D= dextro-[right] rotatory.)asparagine2 AlanineCH3CH33 Zwitterions●In aqueous solution, amino acids areable to form ions that carry bothpositive and negative charge. Suchions are called zwitterion s. They formby the loss of a proton from thecarboxylic acid group and the gain of aproton on the amine group.COOHCH2 NCH2 NHL-alanineHCOOHD-alanine4 Proteins●Protein s are polymers consisting oflong chains of amino acids.
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