Chemistry of Alkenes and Alkynes: Properties, Synthesis and Nomenclature - Prof. Sushama A, Study notes of Organic Chemistry

Download Chemistry of Alkenes and Alkynes: Properties, Synthesis and Nomenclature - Prof. Sushama A and more Study notes Organic Chemistry in PDF only on Docsity! CHEM 2370 Solomons Chapter 7 (9th ed) DANDEKAR CHAPTER 7 ALKENES & ALKYNES: PROPERTIES, SYNTHESIS - INTRODUCTION - NOMENCLATURE: “ E – Z ” SYSTEM - STABILITY : ΔH° OF – HYDROGENATION – COMBUSTION - SYNTHESIS OF ALKENES - FROM: – R-X – R-OH - SYNTHESIS OF ALKYNES - FROM: – -CHX-CHX- – -CH2-CX2- – TERMINAL ALKYNES OF LOWER MW - HYDROGENATION OF ALKENES - HYDROGENATION OF ALKYNES 1 CHEM 2370 Solomons Chapter 7 (9th ed) DANDEKAR INTRODUCTION: > C = C < – C ≡ C – ALKENE ALKYNE C n H 2n C n H 2n - 2 - HYBRIDIZATION ? - BOND ANGLES ? - POLARITY ? SO ? - SOLUBILITY IN: – WATER ? – BENZENE ? – HEXANE ? - DENSITY ? NOMENCLATURE: “ E – Z ” SYSTEM ( REALLY ! ) ☺ - BASIC NOMENCLATURE RULES: REVIEW ( CHAPTER 4 ) HOWEVER, MANY TIMES, THESE RULES ARE NOT ENOUGH ! - WHY ? 2 CHEM 2370 Solomons Chapter 7 (9th ed) DANDEKAR STABILITY : ( RELATIVE ! ) - SEE FIG. 7.1, 7.2 p. 272 - HEATS OF HYDROGENATION NOTE: HYDROGENATION MUST RESULT IN THE FORMATION OF THE SAME PRODUCT(S) i.e. , ALKENES BEING COMPARED MUST HAVE IDENTICAL CARBON SKELETONS ( POSITION OF C=C MAY BE DIFFERENT ) - WHAT IF THE PRODUCTS OF HYDROGENATION ARE NOT IDENTICAL? - CAN USE HEATS OF COMBUSTION (FOR STABILITY COMPARISONS) - HOW COME? COMBUSTION RESULTS IN THE FORMATION OF THE SAME PRODUCT(S) ( = ? ) EVEN IF ALKENES BEING COMPARED DO NOT HAVE IDENTICAL CARBON SKELETONS 5 CHEM 2370 Solomons Chapter 7 (9th ed) DANDEKAR IN GENERAL: STABILITY DEPENDS ON DEGREE OF SUBSTITUTION - SEE p. 273 - MONOSUBSTITUTED ALKENES - DISUBSTITUTED ALKENES (3 TYPES) - TRISUBSTITUTED ALKENES - TETRASUBSTITUTED ALKENES - DO PROBLEMS: 7.2, 7.3, 7.4 p. 273 - CYCLOALKENES - SEE FIG. 7.3 – 7.5 p. 274 - CIS v. TRANS : RING SIZE ? 6 CHEM 2370 Solomons Chapter 7 (9th ed) DANDEKAR SYNTHESIS OF ALKENES: - SEE p. 274 H C C X ? ? H C C OH C C - DEHYDROHALOGENATION OF: R – X - STRONG BASE - HIGH TEMPERATURE ( WHY ? ) ( WHY THESE CONDITIONS ? ) FACILITATE E2 ! (WHY ?) - REGIOCHEMISTRY: - LOCATION OF DOUBLE BOND IN PRODUCT SOMETIMES, MORE THAN ONE PRODUCT IS POSSIBLE ! 7 CHEM 2370 Solomons Chapter 7 (9th ed) DANDEKAR LEAD TO THE MOST STABLE ALKENE - WHY ? STEREOCHEMISTRY OF E2 REACTIONS: - H & X MUST BE COPLANAR - WHY ? - SEE p. 296 LOWER ENERGY TRANSITION STATE FOR DEVELOPING π BOND ALSO, - PREFERABLY , - H & X MUST BE “ ANTI ” - IF “ ANTI ” NOT POSSIBLE, “ SYN ” IS OK ( BUT SLOW : WHY ? ) - WHAT ABOUT “ GAUCHE ” ? - NOT COPLANAR ! - EXPERIMENTAL EVIDENCE ? - SEE e.g. p. 279 10 CHEM 2370 Solomons Chapter 7 (9th ed) DANDEKAR - DO PROBLEMS: 7.7 , 7.8 p. 280 7.26 p. 307 DEHYDRATION OF: R – OH C H C OH C C + HOH CONC. H2SO4 - OBSERVED RELATIVE EASE OF ELIMINATION: 3° > 2° > 1° ( WHAT ABOUT – CH3 ? ) IMPLICATIONS ? - A REASONABLE EXPLANATION : - E1 PROCESS ( FOR 2°, 3° ROH ) - SEE p. 283 - IS H2O A GOOD LEAVING GROUP ? (STEP 2 ) ( v. OH – ) - ROLE OF HA AS A CATALYST - SEE FIG. 7.7 p. 283 - 1° ROH : - E1 NOT LIKELY ( WHY ? ) - SEE p. 284 11 CHEM 2370 Solomons Chapter 7 (9th ed) DANDEKAR NOTE: STEP 1 MUST OCCUR BEFORE E1 / E2 PROCESS: - WHY ? CARBOCATION REARRANGEMENTS: - 1,2 – MIGRATION - OF WHAT? - H: – (HYDRIDE) - R: – (ALKANIDE) - HOW ? - MIGRATING GROUP TAKES THE BOND PAIR WITH IT ( cf. LEAVING GROUPS ) - WHY ? - TO PRODUCE MORE STABLE C+ - SEE p. 285-286 - SEE e.g. p. 286 - REARRANGEMENT IS TYPICALLY SPONTANEOUS SO ? CANNOT BE PREVENTED ! - DO PROBLEMS: 7.10 – 7.12 p. 286-287 12 CHEM 2370 Solomons Chapter 7 (9th ed) DANDEKAR - SOLVENT : NH3 (l) ( OR MINERAL OIL: INERT MEDIUM ) - REACTION TEMPERATURE ? - WITH NH3 (l) ? - WITH MINERAL OIL ? - STARTING MATERIALS : - VIC – DIBROMIDE: – CHBr – CHBr – - PREPARED FROM: – CH = CH – - HOW ? - ADDITION OF Br2 - GEM – DICHLORIDE: – CH2– CCl2 – - PREPARED FROM: H C C OH - HOW ? - ADDITION OF PCl5 - DO PROBLEM : 7.13 p. 290 15 CHEM 2370 Solomons Chapter 7 (9th ed) DANDEKAR - FROM TERMINAL ALKYNES OF: LOWER MW - SN2 REACTION OF: R-X WITH ALKYNIDE ( – C ≡ C: – ) – C ≡ C – R PRODUCT HAS HIGHER MW NOTE: - GOOD YIELDS WHEN: R = CH3 , 1° BUT NOT WHEN: R = 2° , 3° WHY ? - IS ALKYNIDE A STRONG BASE ? ( CHECK pKa ) ( IN ADDITION TO BEING A STRONG NUCLEOPHILE ) SO ? IF: R = 2° , 3°, THEN: E2 WOULD PREDOMINATE, ( RATHER THAN SN2 ) - TO GIVE ? [ SEE TABLE 6.7 , p. 278 ] - DO PROBLEMS : 7.14, 7.15 p. 290 – 291 16 CHEM 2370 Solomons Chapter 7 (9th ed) DANDEKAR - HYDROGENATION OF ALKENES: - H2 Ni HH sp2 sp3 - REACTION TYPE ? - STEREOCHEMISTRY : “SYN ” - REACTION OCCURS ON SURFACE OF CATALYST - SEE FIG. 7.10 p. 294 - CATALYST IS USUALLY “ FINELY DIVIDED ” ( - WHY ? ) - CATALYST ACTION - SEE FIG. 7.9 p. 294 “ TUNNELING EFFECT ” - ALTERNATE LOWER [ ΔG ]‡ PATHWAY SO ? REACTION = FASTER OR SLOWER ? 17