Lecture 1- Dec 16 (1) PDF - Organic Chemistry

Reactive Intermediates, Structure And Reactivity of Aliphatic Compounds Prof.W.S.Hemalika 8L +T Learning Outcomes At the end of the lecture series students should be able to 1. Identify the functional groups on structures of organic compounds 2. Predict the produ...

Reactive Intermediates, Structure And Reactivity of Aliphatic Compounds Prof.W.S.Hemalika 8L +T Learning Outcomes At the end of the lecture series students should be able to 1. Identify the functional groups on structures of organic compounds 2. Predict the products of different reactions of aliphatic compounds 3. Apply the reactivity in defining the synthetic pathways for target aliphatic compounds. References 1. L.G.Wade, Organic Chemistry, Prentice Hall, New Jercy, 2002 2. J.E.McMurry, Fundamentals of Organic Chemistry, Cenage, Boston, 2010 3. M.G. Moloney, Structure and Reactivity in Organic Chemistry, Wiley-Blackwell, New York, 2008  Alkanes molecular formula C n H (2n + 2) Structure, Shape, Hybridization? methane,ethane, propane, diamond, C-nanotubes Home work 1. How the melting point and boiling point of alkanes change with their structure? 2. Explain heat of combustion of alkanes and uses of it. 3. Explain the mechanism of halogenation of alkanes.  Unsaturated Aliphatic Hydrocarbons Alkene: contains a carbon-carbon double bond and has the general formula CnH2n :diene, triene, polyene H H C C H H Ethylene (an alkene) Alkyne: contains a carbon-carbon triple bond and has the general formula CnH2n-2 H-C C-H Acetylene (an alkyne).  2pz sp2 sp2 sp2 Structure of Alkenes A double bond consists of – sigma bond => LINEAR overlap of sp2 hybrid orbitals – pi bond => SIDEWAYS overlap of parallel 2p orbitals p – the two carbon atoms ,four atoms bonded =>lie in a plane, with bond angles 1s of approximately 120° p-orbital sbond sp2-orbital s-orbital top view: H 120° H HC C H 120° sp2-orbital pbond sp2-orbital H C C H 1.10 Å H H1.34 Å Ethylene (an alkene)  Characteristic Reactions of Alkenes Hydrochlorination Oxymercuration H OH H2 O C C + HCl C C C C + Hg ( OAc ) 2 C C Cl H Hg OA c Hydration Hydroboration H H+ C C + H2 O C C C C + BH3 C C OH H BH2 Brominati on Diol formation (oxi dation) Br C C + Br2 C C C C + OsO 4 C C H Br OH OH Bromohydri n formation Hydrogenation (reduction) Br + Br2 H2 O C C + H2 C C C C C C OH H H Electrophilic Addition Reactions Question: Write down one example for each of the above addition reactions Addition Reactions - syn versus anti The elements of X and Y to a double bond can be added from the same side (syn) or from opposite sides (anti). YX X Newmann projections Y  Question HX CH 1. Write down the product of the 3CH reaction following CHCH3 CH3CH C X H HX CH3CH CHCH3 HX =CHCH CH3CH HCl, 3HBr, HI solvent: X H 2OCH2CH3 (Et2O CH3CH or H3C H HX = HCl,HHBr, 3C CH3COH23H HHI C (HOAc, H acet HBr Br H H Br 2. H H solvent: H H CH3CH2OCH 2CH3 (Et2O, ether) H H not formed or only product Explain CH3CO2H (HOAc, acetic acid) Markovnikov’s rule: H+ adds to the carbon of the double bond having the greater number of hydrogens  Carbocation stability governs orientation of the addition H H CH3C CH2 CH3C CH2 H H Br H Br- not formed CH3C CH2 1° carbocation H Br H H CH3C CH2 CH3C CH2 H Br H Br- 2° carbocation product formed Markovnikov’s rule: H+ adds to the carbon of the double bond having the greater number of hydrogens Regioselectivereaction: an addition or substitution reaction in which one of two or more possible products is formed in preference to all others that might be formed Reactive Intermediates Carbocations : C+ positively charged C Free racicals : R Carbanions : C- Carbeens: C  Carbocations + R R C R sp2  Carbocation Stability – relative stability H H CH3 CH3 H C+ CH3 C+ CH3 C+ CH3 C+ H H H CH3 Methyl Ethyl Isopropyl t ert-Bu tyl cation cation cation cation (meth yl) (1°) (2°) (3°) Increasin g carbocation s tability – methyl and primary carbocations are so unstable that they are never observed in solution  Carbocation Stability – the alkyl groups attached to the positively charged carbocations are electron-releasing and thereby help stabilize the positive charge of the cation… – this electron-releasing ability of alkyl groups by (1) the inductive effect – (2) hyperconjugation  Carbocation Stability The inductive effect – the electron-deficient carbon bearing the positive charge polarizes electrons of the adjacent sigma bonds toward it – the larger the volume over which the positive charge is delocalized, the greater the stability of the cation + H3 C + C + CH3 H3 C +  Carbocation Stability Hyperconjugation – partial overlap of the s bonding orbital of an adjacent C-H bond with the vacant 2p orbital of the cationic carbon delocalizes the positive charge