ࡱ> ac`%` UbjbjNN 8Z,,&FFFFFFFZbbbbt$Z:"+,W}!!!!!!!$,#h%!QF!FF!FF}!}!FF qbgI!4 "0:"&&8&F!(or\ooo!! ooo:"ZZZ>$ZZZ>ZZZFFFFFF Unit 2.11 Mechanisms 2.11a Classifying reactions electrophilic addition reactions A typical example of these is between an alkene and a halogen or hydrogen halide. E.g. ethene and hydrogen bromide. They involve an attack by an ion or group with a positive or partial positive charge called an electrophile. In the above reaction the electrophile is a hydrogen ion H+. Electrophiles attack centres of negative charge like the electrons in a double bond. These are addition reactions because the two reactants are added together to give one product. E.g. C2H4 + Cl2 ---> CH2ClCH2Cl elimination reactions A typical example is the formation of an alkene from an alcohol. E.g. Ethanol reacting to form ethene. They involve atoms or groups from either side of a single carbon to carbon bond being removed (eliminated) to form a double bond. E.g. CH3CH2OH ---> C2H4 + H2O Conditions: Heat under reflux with ethanol as solvent. Use KOH here. Reaction type: elimination CH3CH2Br -------------> CH2CH2 + HBr free radical substitution reactions A typical example of these is between an alkane and a halogen. E.g. methane and chlorine. They involve an attack by an atom or group with an unpaired electron called a free radical. In the above reaction ultraviolet light causes chlorine atoms (free radicals) to form, Cl. and methyl radicals, CH3. are also involved. The dot shows the unpaired electron. CH4 + Cl2 ( CH3Cl + HCl nucleophilic substitution reactions A typical example of these is between an alcohol and hydroxide ions. E.g. ethanol and aqueous sodium hydroxide. They involve an attack by an ion or group with a negative or partial negative charge called a nucleophile. In the above reaction the hydroxide ion OH- is the nucleophile. A nucleophile attacks a centre of positive charge such as the partial positive charge on the carbon atom holding a halogen CD+-X. It is a substitution reaction because the attacking species replaces the species originally attached to the carbon atom under attack. E.g. CH3CH2Br + OH- ---> CH3CH2OH + Br- Reagents: concentrated ammonia (in excess) in ethanol solution. Conditions: heat in a sealed tube. reaction type: nucleophilic substitution Mixtures of amines are formed: C2H5I + NH3 ---------> C2H5 NH2 + HI C2H5OH + PCl5 -----> C2H5Cl + POCl3 + HCl 3C2H5OH + PI3 ---> 3C2H5I +H3PO3 oxidation reactions These reactions involve the loss of hydrogen or the gain of oxygen. An oxidising agent like acidified potassium dichromate (VI) is used. E.g. CH3CH2OH + O ---> CH3CHO + H2O CH3CH2OH + 2O ---> CH3COOH + H2O 5CH2 = CH2 + 2H2O + 2MnO4- + 6H+ -----> 5HOCH2CH2OH + 2Mn2+ manganate (VII) Combustion reactions are also in this category. reduction reactions These involve a reaction with hydrogen. A compound gains hydrogen. This can be hydrogen H2 or from a reducing agent like lithium aluminium hydride LiAlH4 in ether or aqueous NaBH4.(sodium borohydride) E.g. CH3CHO + 2H ---> CH3CH2OH 2CH3OH + 2Na ( 2CH3ONa + H2 hydrolysis reactions These all involve a reaction with water. E.g. CH3CH2Br + H2O ---> CH3CH2OH + HBr polymerisation reactions These reactions involve the joining of small molecules called monomers to make large molecules called polymers. A large number (n) of monomer molecules in involved and a long polymer molecule is formed. E.g. nC2H4 ---> -(C2H4)n- Task 2.11a List all reaction types and give one new example of each 2.11b Bond breaking Homolytic fission results in the equal distribution of electrons from a bond between the joining atoms. E.g. Cl:Cl ( Cl. + Cl. The particles formed are free radicals. Heterolytic fission results in both electrons in a bond going to one of the joined atoms. E.g. H:Cl ( H+ + :Cl- The particles formed are ions. The positive ion can accept a lone pair of electrons so is an electrophile. The negative ion can donate a lone pair of electrons so is a nucleophile. Task 2.11b Redraw the above changes to show the movement of electrons with arrows. 2.11c Free radicals electrophiles and nucleophiles A chemical species can be an atom, molecule or part of a molecule. A free radical is a chemical species with an unpaired electron. An electrophile is a chemical species that accept a lone pair of electrons. A nucleophile is a chemical species that can donate a lone pair of electrons. Task 2.11c List examples of free radicals, nucleophiles and electrophiles showing unpaired or lone pairs of electrons. 2.11d Reasons for classifying reagents A chemical reagent will be or produce free radicals, electrophiles or nucleophiles. Classifying these reagents helps identify the type of attack likely in a reaction and so helps predict its mechanism. 2.11e Bond polarity and mechanisms A polar bond forms between two atoms of different electronegativities. E.g. hydrogen and chlorine. +H-Cl - The above charge separation leads to the formation of a hydrogen ion which is an electrophile and can lead to an electrophilic addition mechanism. A non-polar bond forms between atoms of the same electronegativity e.g. in chlorine Cl-Cl. If broken one electron from the bond goes to each atom so creating radicals enabling free radical substitution. Task 2.11e Identify polar and non-polar bonds in the following and suggest mechanisms for their reactions with organic molecules: H2O, Br2, NH3, I2, ICl. 2.11f Heterolytic, nucleophilic substitution SN1 and SN2 mechanisms A nucleophile is a species which attacks a carbon atom with a partial positive charge by donating an electron pair. An example of a nucleophilic substitution is provided by the hydrolysis of 2-methyl-2-bromopropane, to form 2-methylpropan-2-ol. The hydroxide ion is a nucleophile. This is known as nucleophilic substitution. The bromide ion is known as the leaving group. (CH3)3CBr + NaOH ------> (CH3)3COH + NaBr Rate studies of this type show that; Rate = k[R-Br] R=the alkyl group This is an SN1 mechanism with the following steps:  INCLUDEPICTURE "http://www.drbateman.net/asa2sums/sum5.3/arrow2.gif" \* MERGEFORMATINET  (CH3)3C-Br ------> (CH3)3C+ + Br- slow  INCLUDEPICTURE "http://www.drbateman.net/asa2sums/sum5.3/arrow.1.gif" \* MERGEFORMATINET  (CH3)3C+ + OH - ------> (CH3)3COH fast The reaction is thus first order with respect to R-Br but zero order with respect to OH-. First order kinetics is good evidence that the rate determining step is unimolecular. The reaction is therefore given the symbol SN1. Since the nucleophile is not involved in the rate determining step, the mechanism must involve at least two steps. The reaction between bromomethane and hydroxide ion is another nucleophilic substitution reaction. CH3Br + OH - -----> CH3OH + NaBr The rate equation is Rate = k[CH3Br][OH-] In this case the reaction is first order with respect to both [CH3Br] and [OH-]. It is thus second order overall. This is a bimolecular reaction. The reaction is thus given the symbol SN2. It is thought to proceed in a single step involving a transition state.  INCLUDEPICTURE "http://www.drbateman.net/asa2sums/sum5.3/arrow2.gif" \* MERGEFORMATINET  HO - + CH3------Br --------> [ HO - - - CH3 - - - Br] -  INCLUDEPICTURE "http://www.drbateman.net/asa2sums/sum5.3/arrow2.gif" \* MERGEFORMATINET  [ HO - - - CH3 - - - Br] -----------> HO-----CH3 + Br - The cyanide ion CN- can also take part in nucleophilic substitution  HYPERLINK "http://www.drbateman.net/asa2sums/sum5.3/nucsub.htm" See an animation of hetrolytic nucleophilic substitution  HYPERLINK "http://www.drbateman.net/asa2sums/sum5.3/orgmechs.ppt" Presentation of all mechanisms - (needs Powerpoint) 5.3(b)(v).1 Describe an SN1 mechanism and give a rate equation for the reaction of 2-methyl-2-iodopropane with aqueous potassium hydroxide. 5.3(b)(v).1 Describe an SN2 mechanism and give a rate equation for the reaction of bromoethane with aqueous hydrogen cyanide. 5.3(b)(v).1 Describe a mechanism for the reaction of 2-iodopropane with aqueous sodium hydroxide if the rate equation is rate = k[2-iodopropane]. 2.11g The ozone layer Ozone is found in the upper atmosphere where it is formed naturally by the action of UV light on oxygen. 3O2 ( 2O3 (in UV light) The formation mechanism involves free radicals. O=O ( 2O. O2 + O. ( O3 It is valuable here as it prevents much UV light reaching the surface and so reducing skin cancer. Ozone is a problem as it is also a greenhouse gas and near the ground is a lung irritant. Ozone can be destroyed in reactions with other radicals e.g. from CFCs. CCl2F2 ( .CClF2 +Cl. Cl. + O3 (ClO + O2 ClO + O3 ( Cl. + 2O2 See p228/229 Edexcel AS Chemistry Ozone increased by NOx  HYPERLINK "http://www.sciencemuseum.org.uk/antenna/flying/contribution/nitrogen_oxides/" http://www.sciencemuseum.org.uk/antenna/flying/contribution/nitrogen_oxides/ NO2 + sunlight  INCLUDEPICTURE "http://www.grida.no/climate/ipcc/aviation/images/r_arr.gif" \* MERGEFORMATINET  NO + OO + O2  INCLUDEPICTURE "http://www.grida.no/climate/ipcc/aviation/images/r_arr.gif" \* MERGEFORMATINET  O3  Ozone decreased by NOx  HYPERLINK "http://books.google.co.uk/books?id=CQ2fWv9CKLkC&pg=PA42&lpg=PA42&dq=ozone+nitrogen+oxides+aircraft&source=bl&ots=7ZA-M2WnVL&sig=xmRMEAxMhcSnhZm3IfIwDpwOrxA&hl=en&ei=TBCtSZuaJuCGjAfCraCgBg&sa=X&oi=book_result&resnum=9&ct=result#PPA43,M1" 2Sq r 2 3 4 5 : ; C D H I L b U V X Y c d e f j k  + , D E F οh"h"CJH*aJh"CJaJhi8ACJHH*aJHhi8ACJH*aJhi8ACJH*aJhi8ACJaJhi8A5B*CJ\aJphfhi8Ah& P5CJaJmH sH h& PCJaJmH sH h& Phq'PmH sH h& Ph& PmH sH 22L Cc]x&';S ]^`gdCMgdCMgdi8Agdi8Agd$$a$gd& PU *,02@BRTXZhjlBbcdefgmn{|}־ֱ֨֨־֨֨־yqqhqhqhqhqhCMCJH*aJhCMCJaJh hCMCJaJhCMCJaJmH sH hxjhCMCJaJmH sH hCMCJH*aJhi8ACJH*aJhi8ACJH*OJQJaJhi8ACJH*aJhi8A5B*CJ\aJphfhi8ACJaJh"h"CJH*aJh"CJaJ jh"h"CJaJ(}}ļĭhi8ACJH*aJhCM5B*CJ\aJphfhi8ACJaJhi8A5B*CJ\aJphfhi8AhxBhCMCJH*aJhxBhCMCJaJhCMCJaJhCMCJH*aJ?-\]q   %&(<DEUVXY[\_`ijnovwxyû|û" jh`hCMCJaJmH sH h`hCMCJH*aJmH sH hCMCJaJmH sH h]CJaJhi8ACJH*aJhi8ACJaJhi8A5B*CJ\aJphfh"hi8ACJaJh"CJaJhCMCJH*aJhCMCJaJhCMCJH*aJ0&';CDƳzhVVz"hCqWhCqW5CJ4H*aJ4mH sH " jhCqWhCqWCJaJmH sH hCqWhCqW5CJ(aJ(mH sH hCqWCJaJmH sH hCqWh& P5CJaJmH sH h05CJaJmH sH %h0h0B*CJaJmH phsH (h0h05B*CJaJmH phsH hi8Ahi8A5B*CJ\aJphfhi8ACJaJhi8ACJH*aJ!DGHJKNOQR ]^%&-LM2xfVFV:hxCJaJmH sH hxhx5CJaJmH sH hxh& P5CJaJmH sH "hCqW5B*CJaJmH phsH hCqWh& P5CJaJmH sH %hCqWhCqWB*CJaJmH phsH (hCqWhCqW5B*CJaJmH phsH "hCqWhCqW5CJ$H*aJ$mH sH hCqWhCqW5CJ(aJ(mH sH hCqWhCqWCJH*aJmH sH " jhCqWhCqWCJaJmH sH hCqWCJaJmH sH %&M02xV> b#$r&((**+b+m+|+:,,, gd]},gd0gd$gdCqW2xBFNPTV=> HhӸ㠔l\lGlGlGlGl(h4nh4nB*CJH*aJmH phsH hqB*CJaJmH phsH %h4nh4nB*CJaJmH phsH (h4nh4n5B*CJaJmH phsH h4nCJaJmH sH hkCJaJmH sH h'~CJaJmH sH hxCJaJmH sH hlmhlmCJaJmH sH hlmhlmCJH*aJmH sH hlmCJaJmH sH hkh& P5CJaJmH sH a!b!c!d!z!{!|!}!!!""t"w"{"|"}"~"""""""""""$#%#&#'#+#,#-#.#/#0#9#;#F#G#H#I#a#b###?$@$ʶʪʪʡʡʪʪʡʡʑʡʶh0jh0CJUaJh0CJH*aJjh0CJUaJh0:CJH*aJh0CJH*aJh0CJaJ"h0h& P5CJH*aJmH sH h05CJ\aJh0h& P5CJaJmH sH 9@$$$%%&%'%1%2%^%_%e%f%%%%%$&%&p&w&x&&&&&&&&&&' ' '(')'''''''''''''''((F(G(((((((((((h00JCJaJj h0CJUaJjh0CJUaJj> h0Ujh0Uh0:CJH*aJh0CJH*aJh0CJH*aJh0h0CJaJ?()))))Z)e)))))))))* * **=*H******+++ +!+0+a+b+f+g+j+ֶֺֺֺֺ||n| jh:.1h:.1CJaJh:.1CJaJ jh]},h]},CJaJh]},h]},CJH*aJh]},CJaJh]},h& P5CJaJmH sH h0h00JB*CJaJphh0B*CJH*aJphh0B*CJaJphh00J5B*CJaJphh05B*CJaJph%j+k+n+p+s+t+v+w+x+y+z+{+|++9,:,,,,,,,,,,,,,,,,,,,,,,ȴȴȦȘȘȘȁxj jh:.1h}#CJaJh}#CJH*aJh:.1h}#CJH*aJh}#5CJ4H*aJ4h:.1h}#5CJ4H*aJ4 jh}#h}#CJaJhfh}#CJH*aJh]},CJaJh}#CJaJ jh:.1h:.1CJaJh:.1h:.1CJH*aJh:.1CJaJh:.1h:.15CJ4H*aJ4$,,,,,,,,,,,,,,,,,,,-R-T-U-V-------볻vi^Q^hh0JCJ aJ hhCJ aJ hvxhX\0JCJaJ#jnhvxhX\CJUaJhX\hX\CJaJjhX\CJUaJhhX\CJH*aJhX\CJaJh-9CJaJhfCJaJh:.1hf5CJ4H*aJ4 jh}#h}#CJaJhfh}#CJH*aJh}#CJaJh}#h}#CJH*aJ,,,,,-. .......s<kd@$$IfTKk 63%4KaT<kd$$IfTKk 63%4KaT $Ifgd gd]}, --.....%.&.'.(.)................////TTTT媢nl_Wh {CJaJhvxhX\0JCJaJU#jhvxhX\CJUaJhX\hX\CJaJjhX\CJUaJhX\hCJaJhCJaJhX\CJaJjhhCJ UaJ hh0JCJ aJ hCJ aJ j/hhCJ UaJ hhCJ aJ jhhCJ UaJ "http://books.google.co.uk/books?id=CQ2fWv9CKLkC&pg=PA42&lpg=PA42&dq=ozone+nitrogen+oxides+aircraft&source=bl&ots=7ZA-M2WnVL&sig=xmRMEAxMhcSnhZm3IfIwDpwOrxA&hl=en&ei=TBCtSZuaJuCGjAfCraCgBg&sa=X&oi=book_result&resnum=9&ct=result#PPA43,M1 NO2 + sunlight ( NO + O. O3 + NO ( O2 + NO2 Oxides of nitrogen catalyse the decomposition of ozone in this model of atmospheric processes. Task 2.11g Collect and present evidence to show how NOx does or does not affect O3 levels in the atmosphere. .TUUyUzUUU gd gd]},TTTTTUUUUUUUUUUUUzUUUUUUUUhX\hCJaJ hhB*CJH*aJphhhB*CJaJph hh5B*CJaJphhhCJaJhKCJaJh:.1h5CJ4H*aJ4 jhhCJaJhhCJH*aJhCJaJ6&P1h:p Tq/ =!"#$% DdwR  S .Aarrow2b@!G;-Dn@!G;-PNG  IHDR8$PLTEf3̙f3f3ffffff3f3333f333f3f3̙f3̙̙̙̙f̙3̙ffffff3f3333f333f3̙f3̙̙f3̙f3ff̙ffff3f33̙33f333̙f3ffffff3ffff̙fff3fffffff3ffffffffffff3fff3f3f3f3ff33f3ffffff3f3333f333333̙3f3333333f3333f3f3f3ff3f33f33333333f333333333f333f3̙f3f3ffffff3f3333f333f3wUD"wUD"wUD"ݻwwwUUUDDD"""bKGDH cmPPJCmp0712HsKIDAT(SP k&:A DX,!A[lIAC:.SY"*B*ph"O" 0grame$O!$ 0spelleFV@1F 0FollowedHyperlink >*B* ph8OA8 K subscript1 CJH*aJ&Z2Lq0 9 } ~ 9a+|}op`?S o !!!""9"P""v#w######$%&&y&z&&&000000000000000000000000000000000000000000000000000000000 0 0 0 000000000002Lq0 9 S!""P""v#w#####$%&&&K003K00K00K000Q00Q0K00K00K00K00K00K0 0 pFVK0 0K0 0K0 0K0 0@000ܡ00000K00 @I00 I00 I00 @0$ K00<000 0 @0 0 }D2@$(j+,-TU!"#$%&(,,.U '+Uq {}(*[<P""" #k#m####$$%&CCCCXXXCCX8@0(  B S  ? _Hlt223787827 _Hlt224115078 _Hlt224115079 _Hlt224115104 _Hlt22411510566&@@@@@77& єR* ҔR$ ӔR+ ԔR+ ՔR* ֔Rl* הRl+ ؔR,+ ٔR,, ڔRl& ۔Rl   #"#"+"&  *"-"-"& 9 *urn:schemas-microsoft-com:office:smarttagsState9 *urn:schemas-microsoft-com:office:smarttagsplace8*urn:schemas-microsoft-com:office:smarttagsCity   2?|(4Q]uBH t } K T ] c 3 6 9 G 9DT_nz0<ANS_s ALLWjv\h#/MY@LGQ2=!!""#"*"L"O"$$-&5&&&&2?LWQRYsGJ,qy0 9   " # {XZbdu { !!+"8"%&&33333333333333333333333333333333333V C ] ` a  ~ P"""#$%%&&&Q.8T|l}rM - X\mhH/i([-io0E"c!c"?$X$d$!%.%y+]},/3@0:.1>1F23 4$/4l5H;8-969H9":Y:q <x<O> P>i8AgS4n \?` CZr_%I|AdI\h<KxY40nW?VxyA+/7u,A;$aHSX}^o(W .M"v#w###&@hhhhLWWWW%&@$@<@@UnknownGz Times New Roman5Symbol3& z Arial;Wingdings"qheF3ӦF!F!Fad24d&&2QKP)?& P2Unit 2 David BatemanComputer Resources ManagerOh+'0 $0 P \ h tUnit 2David Bateman Normal.dotComputer Resources Manager5Microsoft Office Word@U@Pso@&@p!՜.+,D՜.+,4 hp|  F&' Unit 2 Titled 8@ _PID_HLINKSAG3http://books.google.co.uk/books?id=CQ2fWv9CKLkC&pg=PA42&lpg=PA42&dq=ozone+nitrogen+oxides+aircraft&source=bl&ots=7ZA-M2WnVL&sig=xmRMEAxMhcSnhZm3IfIwDpwOrxA&hl=en&ei=TBCtSZuaJuCGjAfCraCgBg&sa=X&oi=book_result&resnum=9&ct=result PPA43,M1#JMhttp://www.sciencemuseum.org.uk/antenna/flying/contribution/nitrogen_oxides/UY6http://www.drbateman.net/asa2sums/sum5.3/orgmechs.pptUw  4http://www.drbateman.net/asa2sums/sum5.3/nucsub.htmU  !"#$%&'()*+,-/0123456789:;=>?@ABCDEFGHIJKLMNOQRSTUVWYZ[\]^_bRoot Entry F' qdData .1Table<&WordDocument8ZSummaryInformation(PDocumentSummaryInformation8XCompObjq  FMicrosoft Office Word Document MSWordDocWord.Document.89q