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CHAPTER 7 NOTES - CHAPTER 7 ALKENES ALKYNES PROPERTIES...

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CHAPTER 7 ALKENES & ALKYNES: PROPERTIES, SYNTHESIS - - INTRODUCTION - - NOMENCLATURE: E – Z ” SYSTEM - - STABILITY : H ° OF HYDROGENATION COMBUSTION - - SYNTHESIS OF ALKENES - - FROM: R- X R- OH >C X -C X < - - SYNTHESIS OF ALKYNES - - FROM: -C H X -C H X - -C H 2 -C X 2 - TERMINAL ALKYNES OF LOWER MW - - HYDROGENATION OF ALKENES - - HYDROGENATION OF ALKYNES - - INDEX OF HYDROGEN DEFICIENCY
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INTRODUCTION: > C = C < – C C – ALK E NE ALK Y NE 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 ? - - ROTATION ABOUT >C =C< IS RESTRICTED ( NOT FREE ! ) SO ? v GEOMETRIC ISOMERS MAY BE POSSIBLE: vi ( DIASTEREOMERS ) SO ? - - NEED A SYSTEMATIC WAY TO DISTINGUISH BETWEEN THEM !
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- - DI SUBSTITUTED ALKENES : CIS , TRANS CIS - TRANS – ( SAME SIDE ) ( OPPOSITE SIDE ) - - TRI SUBSTITUTED ALKENES: o o CANNOT USE “ CIS OR TRANS ” TO DISTINGUISH BETWEEN THESE SO ? NEED MORE SPECIFIC NAME USE “ E – Z ” SYSTEM ! HOW ? USE: CAHN – INGOLD – PRELOG RULES - - AT EACH sp 2 – C , PRIORITIZE THE TWO GROUPS ACCORDING TO ATOMIC NUMBER AS: ( I ) & ( 2 ) HIGHER AT. NO. = HIGHER PRIORITY = 1 LOWER AT. NO. = LOWER PRIORITY = 2 - - THEN DETERMINE WHETHER THESE PRIORITIES ARE ARRANGED R H R H H R R H H H 3 C Cl Br H Br Cl H 3 C
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CIS ” – LIKE : “ Z ISOMER OR TRANS ” – LIKE : “ E ISOMER - - DO PROBLEM : 7.1 p. 289
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STABILITY : ( RELATIVE ! ) - - SEE FIG. 7.1, 7.2 p. 290, - - 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 ) - - HEATS OF COMBUSTION - - SEE p. 291 NOTE: COMBUSTION RESULTS IN THE FORMATION OF THE SAME PRODUCT(S) EVEN IF ALKENES BEING COMPARED DO NOT HAVE IDENTICAL CARBON SKELETONS IN GENERAL: STABILITY % DEGREE OF SUBSTITUTION - - DO PROBLEMS : 7.2, 7.3, 7.4 p. 291 - - CYCLOALKENES - - SEE FIG. 7.3 – 7.5 p. 292 - - CIS v. TRANS : RING SIZE ? SYNTHESIS OF ALKENES:
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- - SEE p. 292 - - DEHYDROHALOGENATION OF : R – X - - STRONG BASE - - HIGH TEMPERATURE ( WHY ? ) ( WHY THESE CONDITIONS ? ) FACILITATE E2 ! C C X H C C OH H C C Br Br ? ? ?
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- - REGIOCHEMISTRY : - - LOCATION OF DOUBLE BOND IN PRODUCT - - SOMETIMES, MORE THAN ONE PRODUCT IS POSSIBLE ! SO ? HOW TO PREDICT THE OUTCOME OF THE REACTION ? i.e. , MAJOR / MINOR PRODUCTS ? I β β β H H H β β H H β β H H β β H H
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- - DEPENDS ON SIZE OF BASE: - - SMALL BASE: ZAITSEV’S RULE - - MAJOR PRODUCT = MOST STABLE ALKENE - - SEE e.g. p. 294 - - DO PROBLEM : 7.5 p. 295 - - BULKY BASE: HOFMANN RULE - - MAJOR PRODUCT = LEAST STABLE ALKENE - - SEE e.g. p. 295 - - WHY ? - - CANNOT ABSTRACT THE HYDROGEN THAT WOULD LEAD TO THE MOST STABLE ALKENE - - WHY ?
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- - 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 ” ?
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