ORGANIC CHEMISTRY- 1

Transcription

1 ORGANIC CEMISTRY- 1

2 ALKENES Alkenes are also called Olefins (C n 2n ) unsaturated hydrocarbons. Alkenes occur abundantly in nature. Ethylene ( 2 C=C 2 ) is a plant hormone that induces ripening in fruit. Functional group = carbon-carbon double bond sp2 hybridization => flat, 120 o bond angles σ bond & π bond => 2 C=C 2 No rotation about double bond!

3 Examples: C 3 6 propylene ALKENES C 3 C C 2 Propene C 4 8 butylenes C 3 2 C C C 2 1-Butene C 3 C C C 3 2-Butene 2 C 3 C C C 2 C 3 2-methylbutene

4 Cis/Trans Isomerism: When disubstituted (two substituents other than hydrogen) alkenes contain two substituents on the same side of the double bond they are called cis alkenes and when substituents are attached to opposite side are called trans alkenes. Two methyl groups on the same side of double bond = Cis

5 Cis trans isomerism is not possible If one of the double-bond carbons (vinyl carbon) is attached to two identical groups. C 3 C=CC 3 C 3 C 2 C=C 2 Yes No C3 (C 3 ) 2 C=CC 3 C 3 C=CC 2 C 3 No Yes

6 E and Z Alkenes For trisubstituted (three substituents) and tetrasubstituted (four substituents) double bonds, a more general method is needed for describing doublebond geometry. If the igher mass ranked groups are on the same side, then alkene has Z geometry (German zusammen, meaning together. ) If the higher-ranked groups are on opposite sides, while the alkene has E geometry (German entgegen, meaning opposite).

7 E and Z Alkenes

8 Examples: 3 C C 2 C 3 C C C 3 (Z)-3-methyl-2-pentene or (3-methyl-cis-2-pentene 3 C C C Cl Br (E)-1-bromo-1-chloropropene

9 Stability of Alkenes The trans isomer is more stable than the cis isomer by 2.8 kj/mol (0.66 kcal/mol) at room temperature, corresponding to a 76;24 ratio. Cis alkenes are less stable due to steric strains between two larger substituents on the same side of the double bond.

10 Industrial Preparation of Alkenes Ethylene, Propene and butane are synthesized industrially by cracking of light alkanes. The process is complex and involves radical reactions. The hightemperature reaction conditions cause spontaneous homolytic breaking of C -C and C - bonds and as results smaller fragments are formed.

11 Preparations Dehydration of Alcohols Removal of water is called dehydration. Con. 2 SO 4 is used. C 3 -C 2 -O 2 SO 4 C 2 =C 2 2 O The order of dehydration is ter. alcohol > sec. alcohol > prim. alcohol Unsymmetrical Alcohol secondary or ter. C 3 -C 2 -C-C 3 2 SO 4 C 3 -C=C-C 3 80% O C 3 -C 2 -C=C 2 20% Saytzev: ighly substituted alkenes are formed. 2 O

12 Mechanism for dehydration of an alcohol = E1 1) C C O + C C O 2 2) C C O 2 RDS C C + 2 O 3) C C C C +

13 Saytzeff Product: According to this rule, major product is the most substituted alkene i.e., major product is obtained by elimination of from that β-carbon which has the least number of hydrogen. ofmann Rule : According to this rule, major product is always least substituted alkene i.e., major product is formed from β-carbon which has maximum number of hydrogen.

14 By dehydrohalogenation of Alkyl halide Alcoholic solution of KO R C-C 2 -X KO Alcoholic R C=C 2 2 O KX By Dehalogenation of Vicinal dihalides aving two halogen atoms on adjacent carbon atom is called vicinal dihalides R C-C 2 -X Zn Alcohol R C=C 2 ZnCl 2 X Zn C 3 -C 2 -CBr-C 2 Br C 3 -C 2 -C=C 2 ZnBr 2

15 ydrogenation The addition of hydrogen in the presence of catalyst E.g. Pd (palladium) in BaSO 4. Pd/BaSO 4 C C + 2 C C 3 C C C 2 Pd-CaCO 3 quinoline Lind- Cat 3 C C=C 2

16 Chemical Properties Reduction of Alkenes: ydrogenation Addition of - across C=C Reduction in general is addition of 2 or its equivalent Requires Pt or Pd as powders on carbon and 2 ydrogen is first adsorbed on catalyst Reaction is heterogeneous (process is not in solution) C 2 =C 2 2 Ni C 3 -C 3 2 Ni

17 alogenations Reaction: Bromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process. F 2 is too reactive While I 2 does not add. Mechanism Br C C Br-Br C C Br Bromonium ion Br C C Br Addition is exclusively trans

18 (Addition of X) C C + X C C X Mechanism: X electrophile r.d.s. + Carbonium ion X Flow of a pair of electron fast Note: r.d.s. = rate determining step X

19 Chemical Properties (Mechanism of X Addition Reactions) The first step is electrophilic attack of +, leading to the formation of carbonium ion. This is a rate determining step. It is then followed by the attack at the positive carbon center by X - which is a fast step. Markownikoff s rule means X group attaches to the more highly substituted carbon atom, while the attaches to the less highly substituted carbon.

20 With conc. sulphuric acid Do you remember the Markovnikoff s Rule? What is / are the products of the following? R + -OSO 3 Alcohol Mechanism C C R R -OSO C 2 O C 3 C C C OSO 3 OSO 3 ethyl hydrogen sulphate Protonium ion R R C O

21 Markownikoff s rule The atom will attach to carbon having more no. of atoms. In general, the greater the no. of alkyl grops present, or the larger is the alkyl group, the more stable is the carbonium ion. Stability of carbonium ion: 3 ry C + > 2 ry C + > 1 ry C + > C C 3 C C C -X 3 C 3 C C C Markonikove 3 o More stable 3 C 3 C C C Anti markonikove 2 o less stable

22 22 ydroboration Borane (B 3 ) is electron deficient is a Lewis acid. Borane adds to an alkene to give an organoborane.

23 23 Orientation in ydration via ydroboration Regiochemistry is opposite to Markovnikov orientation (Anti-MarkovniKov addition). and O add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)

24 Mechanism of ydroboration 24 Borane is a Lewis acid Alkene is Lewis base Transition state involves anionic development on B The components of B 3 are added across C=C More stable carbocation is also consistent with steric preferences

25 25 Addition of Carbenes to Alkenes The carbene functional group is half of an alkene Carbenes are electrically neutral with six electrons in the outer shell They add symmetrically across double bonds to form cyclopropanes.

26 Oxidation of Alkene Oxidation of Alkenes with peroxyacid results in acid and Epoxide synthesis.

27 Ozonolysis (Oxidation of Alkene) Bond Breaking) Mechanism

28 Ozonolysis Chemical Properties Products are carbonyl compounds which can be easily identified. If RC=O (aldehyde) will be formed which will be further oxidised by 2 O 2 to organic acid. Since the resulting carbonyl compounds are used for identification, thus oxidation is an unfavorable process. In order to prevent this happens, zinc dust is added.

29 Oxidation a. at room temperature (ydroxylation) (addition) C C + [ O ] + 2 O C C f r o m Mn O 4 - / O - O O b. at vigorous condition (bond breaking) R C C R' + [O] R C O + O C R' In acidic condition, the products will be oxidised to acid or ketone. R Further oxidation O C O

30 Alkynes Introduction Structure and Bonding: Recall that the triple bond consists of 2 bonds and 1 bond. Each carbon is sp hybridized with a linear geometry and bond angles of

31 Alkynes Introduction Structure and Bonding: Estimation of the energy of pi bonds in ethylene (one and one bonds) and acetylene (one and two bonds).

32 Alkynes Introduction Structure and Bonding: Alkynes contain a carbon carbon triple bond. Terminal alkynes have the triple bond at the end of the carbon chain so that a hydrogen atom is directly bonded to a carbon atom of the triple bond. Internal alkynes have a carbon atom bonded to each carbon atom of the triple bond. An alkyne has the general molecular formula C n 2n-2, giving it four fewer hydrogens than the maximum possible for the number of carbons present. Thus, the triple bond introduces two degrees of unsaturation.

33 Alkynes Physical Properties: The physical properties of alkynes resemble those of hydrocarbons of similar shape and molecular weight. Alkynes have low melting points and boiling points. Melting point and boiling point increase as the number of carbons increases. Alkynes are soluble in organic solvents and insoluble in water.

34 Acetylene can be prepared from calcium carbide and water. CaC 2 + O -C C- Calcium carbide is prepared by heating coke and calcium oxide in an electric furnace (~2500 o ).

35 Alkynes Recall that alkynes are prepared by elimination reactions. A strong base removes two equivalents of X from a vicinal or geminal dihalide to yield an alkyne through two successive E2 elimination reactions.

36 Alkynes Alkyne Reactions: Additions Like alkenes, alkynes undergo addition reactions because they contain relatively weak bonds. Two sequential addition reactions can take place: addition of one equivalent of reagent forms an alkene, which can then add a second equivalent of reagent to yield a product having four new bonds.

37 Alkynes ydrohalogenation: Electrophilic Addition of X

38 Alkynes ydrohalogenation: Electrophilic Addition of X Alkynes undergo hydrohalogenation, i.e the, addition of hydrogen halides, X (X = Cl, Br, I). Two equivalents of X are usually used: addition of one mole forms a vinyl halide, which then reacts with a second mole of X to form a geminal dihalide.

39 Alkynes ydrohalogenation: Electrophilic Addition of X

40 Alkynes are reduced to alkanes by addition of 2 over a metal catalyst. The reaction occurs in two steps through an alkene intermediate. Pd, CaCO3/Quinoline

41 Oxidative Cleavage of Alkynes Alkynes are cleaved by powerful oxidizing agent such as KMnO 4 or O 3.

42 Alkynes Reactions of Acetylide anions: Acetylide anions react with unhindered alkyl halides to yield products of nucleophilic substitution. Because acetylides are strong nucleophiles, the mechanism of substitution is S N 2, and thus the reaction is fastest with C 3 X and 1 0 alkyl halides.

43 Alkynes acidity: Terminal alkynes are more acidic than alkenes.

44 Alkynes alogenation: Addition of alogen alogens X 2 (X = Cl or Br) add to alkynes just as they do to alkenes. Addition of one mole of X 2 forms a trans dihalide, which can then react with a second mole of X 2 to yield a tetrahalide.

45 Alkynes alogenation: Addition of alogen

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