N.b. A catalyst is a species which speeds up a chemical reaction but which remains chemically unchanged. Reverse process of dehydration of an alcohol

Transcription

1 An Introduction to Organic hemistry N.b. A catalyst is a species which speeds up a chemical reaction but which remains chemically unchanged. ydration (Addition) Reverse process of dehydration of an alcohol i) 2 SO 4 ii) 2 O 2 = O Reaction with alogens (Addition) Br 2 Br Br BUT if Br 2 is dissolved in 2 O (bromine water) the following occurs:- Br 2 / 2 O O Br 101

2 An Introduction to Organic hemistry Reaction with ydrogen alides (Addition) Br Br The product of this reaction is called a haloalkane or an alkylhalide. But I I 3 1-iodopropane 3 I 3 2-iodopropane Why is 1-iodopropane not formed? 102

3 An Introduction to Organic hemistry The intermediate in the reaction which forms 1-iodopropane is too unstable. This is called a regioselective reaction as only one of the possible products is formed. This is also a special named reaction: Markovnikov addition. Markovnikov Rule In the addition of an acid to a carbon-carbon double bond, the hydrogen of the acid attaches itself to the carbon atom of the double bond with the greater number of hydrogen atoms. Why? Due to the relative stability of the intermediate species in the reaction called carbocations PRIMARY SEONDARY TERTIARY INREASING STABILITY 103

4 An Introduction to Organic hemistry onsider the mechanism of the addition. The first step of the reaction is proton addition: PRIMARY carbocation very unstable SEONDARY carbocation I 3 2-iodopropane 104

5 An Introduction to Organic hemistry Oxidation Like alkanes, alkenes can be burnt in the presence of oxygen O 2 2O O O 2 2O O Of more synthetic use are processes such as ozonolysis and epoxidation. Ozonolysis + O 3 alkene ozone - + O O O O O O ozonide + O O O - O O aldehydes ketones - + O O O Resonance and canonical forms of ozone. Epoxidation - alkene oxidant O epoxide 105

6 An Introduction to Organic hemistry The Alkynes A homologous series of unsaturated compounds with general formula n 2n-2 (where n is an integer greater than 1) that contain a triple bond. ethyne 2 2, propyne 3 4, butyne 4 6, pentyne 5 8, hexyne 6 10, heptyne 7 12, octyne 8 14, nonyne 9 16, decyne 10 18, etc. Industrial Preparation of Ethyne (Acetylene) O 2 +2O 10 2 Laboratory Preparation R R' 2KO R R' + 2KBr O Br Br This process is called dehydrohalogenation. 106

7 An Introduction to Organic hemistry hemistry atalytic Addition of ydrogen (ydrogenation) ompare with the hydrogenation of alkanes Addition of halogens X 2 X 2 X X X X X X Addition of hydrogen halides X X X X X The addition follows the Markovnikov rule. It is also possible to add different halogens l l Br l Br 107

8 An Introduction to Organic hemistry ydration R R' + 2 O + /at R R' O The catalyst is usually mercury(ii) or iron(iii) onversion to Benzene yclic ydrocarbons ycloalkanes u General formula n 2n due to closing the ring but chemistry is typical of alkanes not alkenes. 108

9 An Introduction to Organic hemistry Aromatic ydrocarbons The most important one is benzene 6 6. A planar ring of six carbon atoms in which all the carbon-carbon bonds are equal in length. - = - benzene nm nm nm Therefore the - bonds in benzene are intermediate between single and double bonds... ow might this be explained? Kekulé approach anonical or resonance forms (or in the case of benzene Kekulé forms) 109

10 An Introduction to Organic hemistry ii) Molecular orbital approach Each is sp 2 hybridised with a surplus electron in the p z orbital. These p electrons The delocalised ring or aromatic system is very stable. hemistry ydrogenation For aromatic systems we use: / 2 / Ni catalyst Of greater interest are substitution reactions. All of the examples given here are electrophilic substitution reactions. 110

11 An Introduction to Organic hemistry An electrophile (E + ) is an electron deficient species which is capable of accepting a lone pair of electrons - electron seeking. Nitration NO 2 NO 2 NO 3 >55 alogenation NO 2 + X 2 alogen carrier X + X X 2 is chlorine or bromine The halogen carrier is All 3, Fel 3 or iodine. Friedel-rafts Reactions Very important in synthetic chemistry. There are two main types, both usually require a Lewis acid. 111

12 An Introduction to Organic hemistry Alkylation l All 3 + l The reaction is difficult to stop at this point % 17% 52% The products of the second Friedel-rafts reaction show different substitution about the aromatic ring: ortho, meta and para. 1,2-dimethylbenzene (orthoxylene) 1,3-dimethylbenzene (metaxylene) 1,4-dimethylbenzene (paraxylene) 112

13 An Introduction to Organic hemistry Acylation + R O l All 3 O R + l N.B. If a molecule has multiple functional groups it can display the chemistry of both groups. 2 l l 2 l l hν l 2 l 2 All 3 Organohalogen ompounds l General formula R-X where X is a halogen. If R is an alkyl group then R-X is called an alkyl halide. Alkyl halides are useful reagents in synthesis as the halogen is easily replaced. If R is an aryl group (benzene) then R-X is called an aryl halide. The halogen of aryl halides is difficult to replace. 113

14 An Introduction to Organic hemistry Preparation Via Markovnikov addition of X to an alkene. Br Br From alcohols. 3 2 O + l RO + Pl 5 RO + SOl 2 Znl l + 2 O Rl + POl 3 + l Rl + SO 2 + l Their reactivity arises from the polarity in the carbon-halogen bond δ+ δ Alkyl halides react readily with nucleophiles. A nucleophile is an electron rich species (Lewis base) which is capable of donating a lone pair of electrons. X 114

15 An Introduction to Organic hemistry hemistry Reaction scheme for alkyl halides Used in Friedel-rafts Reactions R-O Alcohol R-O-R' Ether NaO(aq) conc. Reflux RN 2 Primary amine N 3 R-X N - R'-O - (R'-O/Na) Williamson Reaction MO 2 R' (M = Ag + ) RO 2 R' RX R 2 N Secondary amine R-N Nitrile RX R 3 N Tertiary amine R = alkyl group X = halogen R' = alkyl group RX R 4 N + X - Quarternary ammonium salt ALL the reactions shown (except the Friedel- rafts reaction) are nucleophilic substitution reactions. 115

16 An Introduction to Organic hemistry Do they all proceed via the same mechanism? There are two possible routes: S N 1 unimolecular nucleophilic substitution reaction proceeds in TWO steps the slower first rate determining step (RDS) involves only ONE molecule. slow fast Nu X + X Nu Nu sp 2 hybridised planar carbocation 50 : 50 Note there is no control over stereochemistry due to the planar intermediate a racemic mixture is always produced. S N 2 bimolecular nucleophilic substitution reaction proceeds in ONE step involving TWO molecules. Nu X Nu X Nu + X transition state Note the inversion of stereochemistry. 116

17 An Introduction to Organic hemistry ow is one pathway favoured? The S N 1 pathway necessitates the formation of a carbocation. These are unstable species, but may be stabilised by: i) alkyl groups the inductive effect of such groups reduces the positive charge thus stabilising the carbocation. primary secondary tertiary increasing stability ii) solvent choice polar solvents will better stabilise the carbocation. Grignard Reagents Grignard reagents are very useful in synthesis. They are formed from the reaction of organohalogen compounds with magnesium e.g. 3 2 Br + Mg Formally MgBr i.e. nucleophilic carbon. 117

18 An Introduction to Organic hemistry i) React with aldehydes and ketones forming secondary and tertiary alcohols respectively. ii) React with carbon dioxide. RMgBr + O 2 ROO Alcohols ompounds containing the O group. O O O Primary Secondary Tertiary Increasing reactivity O methanol is also a primary alcohol 118

19 An Introduction to Organic hemistry Preparation i) From alkyl halides 3 2 Br + O O + Br - ii) Reduction of aldehydes, ketones and carboxylic acids. What is reduction? In organic chemistry, it can simply be considered as the addition of hydrogen and/or the lessening of oxygen content in a molecule. onversely oxidation is the removal of hydrogen and/or addition of oxygen. ommon reducing agents are hydrides and sources of - (nucleophilic hydrogen) e.g. NaB 4 and LiAl 4. [] symbolises reducing agent [O] symbolises oxidising agent e.g. 3 O + 2[] 3 2 O 119

20 An Introduction to Organic hemistry hemistry Oxidation Primary alcohols are most reactive and susceptible to oxidation yielding aldehydes which in the presence of excess oxidant yield carboxylic acids. Secondary alcohols oxidise to ketones. Tertiary alcohols are very difficult to oxidise since they undergo cleavage. ommon oxidising agents [O] are typically metal based oxidants in which the metal is in a high oxidation state. e.g. i) hromium(vi) in acid: hromium is reduced from r(vi) to r(iii). ii) Manganese (vii) in permanganate (MnO 4 - ) Manganese (vi) in manganate (MnO 4 2- ) Manganese is typically reduced to Mn(iv) or Mn(ii) 120