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Final

CHEM 51LB Final: Comprehensie Practical Study guide - wet lab & mechanisms!


Department
Chemistry
Course Code
CHEM 51LB
Professor
Susan King
Study Guide
Final

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!
WET!LAB!
§ write&down&observations&(color,&smell,&mass,&steps&on&how&to&do&a&technique,&etc.)&and&any&
necessary&calculations&(percent&yield,&percent&recovery,&or&percent&composition).&
o Recrystallization&
o Melting&point&
o TLC&
o Acid-base&extractions&
§ Reflux,&gas&chromatography,&IR,&NMR,&or&any&other&techniques&that&take&a&long&time&to&set&up&
and/or&do&will$not&be&on&the&wet&lab&
DRY!LAB!
§ 1-2&questions&about&predicting&and&drawing&mechanisms&&
o will&give&a&reaction&where&the&compounds&may&look&complicated,&but&if&you&remember&
the&mechanisms&learned&from&this&quarter&you&just&need&to&look&for&the&similarities&and&
apply&them)&
§ 1&question&about&conversions&&
o similar&to&doing&your&reagent&table,&but&just&a&little&bit&trickier&
§ 1-2&critical&thinking&questions&&
o applying&lab&concepts&to&a&hypothetical&situation,&such&as&finding&a&possible&reason&that&
something&went&wrong&in&one&of&your&experiments&
§ 1&question&where&you&predict&a&compound&structure&using&IR&and&NMR&spectra&
LAB!MATHEMATICS!
§ reagent&table&calculations&&
o conversions&
o limiting&reagents&for&theoretical&yield&
o anything&in&the&table&&
§ Percent&yield&=&actual&yield/theoretical&yield&x&100%&
o What&are&some&reasons&that&your&percent&yield&may&be&high&(over&100%)?&What&about&
lower&yield&(below&100%)?&
High:&inadequate&purification&or&filtration&leads&to&presence&of&water&in&solid,&
leading&to&a&high&yield&
Low:&human&error,&evaporation,&some&left&behind&in&purification&vessels&or&in&
filtrate&(if&the&solid&is&water&soluble,&then&rinsing&with&cold&water&may&cause&
some&of&the&product&to&slip&through&and&precipitate&in&the&filtrate)&
§ Percent&recovery&=&amount&of&recrystallized&product/crude&product&x&100%&
§ Percent&composition&=&mass&of&desired&element&or&compound&in&a&mixture/total&mass&of&
compounds&within&mixture&x&100%&
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!
MECHANISMS!!
1. nucleophilic&substitution&
2. bromination&of&an&alkene&
3. acid-catalyzed&dehydration&of&a&primary&and&secondary&alcohol&
4. base-induced&dehydrobromination&of&isomeric&halides&
1. Nucleophilic!Substituting!–!Experiment!2!!
§ two&types&of&nucleophilic&substitution:&SN1&and&SN2&
o When&determining&a&nucleophilic&substitution&reaction,&you&must&look&at&the&type&of&
leaving&group&(1o,2o,3o),&the&nucleophile,&and&the&solvent.&&
o SN1&favors:&
3o&leaving&group&
weak&nucleophile&
polar&protic&solvent&&
o SN2&favors:&&
1o&leaving&group&
strong&nucleophile&
polar&aprotic&solvent&
§ In&the&case&of&Experiment&2,&benzyl&ether&was&synthesized&at&reflux&(boil&a&solution&while&
condensing&the&vapor&continuously&by&cooling&and&returning&it&to&the&flask&that&it&will&be&
reacting&in)&by&nucleophilic&substitution&of&one&of&three&unknown&compounds&(4-
methoxyphenol,&4-chlorophenol,&and&4-tert-butylphenol)&with&benzyl&bromide&in&sodium&
hydroxide&(NaOH—solvent).&&
§ It&is&arguable&whether&the&reaction&for&this&experiment&is&SN1&or&SN2&&
o SN2&can&occur&because&our&reagents&fulfill&the&general&requirements&for&this&reaction.&&
o However,&SN1&is&another&possibility&because&the&primary&carbocation&formed&after&the&
release&of&the&bromine&atom&from&benzyl&bromide&is&stabilized&by&the&benzene&ring.&
The&pi&bonds&on&the&benzene&ring&can&move&their&electrons&around,&which&means&they&
can&also&go&to&an&area&of&positive&charge,&which&is&our&carbocation.&Thus,&the&
carbocation&can&be&maintained&long&enough&for&an&SN1&reaction&to&occur.&&
o know&how&to&draw&both&SN1&and&SN2&mechanisms:&&
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