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Chapter 21 Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution Reactions
Why this Chapter? Carboxylic acids are among the most widespread of molecules. A study of them and their primary reaction “nucleophilic acyl substitution” is fundamental to understanding organic chemistry
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Carboxylic Compounds Acyl group (C=O) bonded to X, an electronegative atom or leaving group X includes: halide (acid halides), acyloxy (anhydrides), alkoxy (esters), amine (amides), thiolate (thioesters), phosphate (acyl phosphates)
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General Reaction Pattern
Nucleophilic acyl substitution
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21.1 Naming Carboxylic Acid Derivatives
Acid Halides, RCOX Derived from the carboxylic acid name by replacing the -ic acid ending with -yl or the -carboxylic acid ending with –carbonyl and specifying the halide Cyclohexanecarboxylic Acid Cyclohexanecarbonyl chloride Acetic Acid Acetyl chloride Benzoic Acid Benzoyl bromide
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Naming Acid Anhydrides, RCO2COR'
If symmetrical replace “acid” with “anhydride” based on the related carboxylic acid Acetic anhydride Benzoic anhydride Succinic anhydride Unsymmetrical anhydrides - cite the two acids alphabetically Acetic benzoic anhydride
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Cyclopentane-carboxamide N,N-Diethylcyclohexanecarboxamide
Naming Amides, RCONH2 With unsubstituted NH2 group. replace -oic acid or -ic acid with -amide, or by replacing the -carboxylic acid ending with –carboxamide Cyclopentane-carboxamide Acetamide Hexanamide If the N is further substituted, identify the substituent groups (preceded by “N”) and then the parent amide N-Methylpropanamide N,N-Diethylcyclohexanecarboxamide
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Cyclohexanecarboxylic acid tert-Butyl cyclohexanecarboxylate
Naming Esters, RCO2R’ Name R’ and then, after a space, the carboxylic acid (RCOOH), replace“-ic acid” with “-ate” Malonic acid Dimethyl malonate Acetic acid Ethyl acetate Cyclohexanecarboxylic acid tert-Butyl cyclohexanecarboxylate
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Naming Thioesters, RCOSR’
Name like an ester but: For common names add “thio” before the acid name “-ate” Methyl acetate Methyl thioacetate Ethyl butanoate Ethyl butanthioate Methyl cyclohexanecarboxylate Methyl cyclohexanecarbothioate For IUPAC names Replace “-oate” or “-carboyxlate” with “thioate” or “-carbothioate”
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Naming Acyl Phosphates
Name Acyl group then add -phosphate: Benzoyl phosphate Acetyl adenosyl phosphate If an alkyl group is attached to a phospate O name it after the acyl group
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Naming Summary:
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Learning Check: Name the following:
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4-methylpentanoyl chloride Isopropyl 2-methylpropanoate
Solution: Name the following: 4-methylpentanoyl chloride Cyclohexylacetamide Isopropyl 2-methylpropanoate
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Learning Check: Name the following:
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Isopropyl cyclopenanecarboxylate Cyclopentyl 2-methylpropanoate
Solution: Name the following: Benzoic anhydride Isopropyl cyclopenanecarboxylate Cyclopentyl 2-methylpropanoate
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Learning Check: Name the following:
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Solution: Name the following: N-Methyl-4-pentenamide
(R)-2-Hydroxypropanoyl phosphate Ethyl 2,3-dimethyl-2-butenethioate
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21.2 Nucleophilic Acyl Substitution
Carboxylic acid derivatives have an acyl carbon bonded to a group Y that can leave A tetrahedral intermediate is formed and the leaving group is expelled to generate a new carbonyl compound, leading to substitution
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Nucleophilic Acyl Substitution
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Relative Reactivity of Carboxylic Acid Derivatives
Nucleophiles react more readily with unhindered carbonyl groups More electrophilic carbonyl groups are more reactive to addition (acyl halides are most reactive, amides are least) The intermediate with the best leaving group decomposes fastest
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Substitution in Synthesis
More reactive acid derivatives can be readily converted into a less reactive one Reactions in the opposite sense are possible but require more complex approaches
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General Reactions of Carboxylic Acid Derivatives
ammonia or an amine an amide alcohols esters hydride source an aldehyde or an alcohol water carboxylic acid Grignard reagent a ketone or an alcohol
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Complete the following reaction:
Learning Check: Complete the following reaction:
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Complete the following reaction:
Solution: Complete the following reaction:
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Learning Check: Rank compounds in the following sets in order of reactivity toward nucleophilic acyl substitution: (#1 = fastest)
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Solution: Rank compounds in the following sets in order of reactivity toward nucleophilic acyl substitution: (#1 = fastest) 1 2 3 1 3 2
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21.3 Reactions of Carboxylic Acids Nucleophilic Acyl Substitution
Must enhance reactivity Convert OH into a better leaving group Specific reagents can produce acid chlorides, anhydrides, esters, amides
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Carboxylic Acids into Acid Chlorides
Reaction with thionyl chloride, SOCl2
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Mechanism of Thionyl Chloride Reaction
Nucleophilic acyl substitution pathway Carboxylic acid is converted into a chlorosulfite which then reacts with chloride
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Carboxylic Acids into Acid Anhydrides
Acid anhydrides can be derived from two molecules of carboxylic acid by strong heating to remove water
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Carboxylic Acids into Esters
Methods include reaction of a carboxylate anion with a primary alkyl halide
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Fischer Esterification
Heating a carboxylic acid in an alcohol solvent containing a small amount of strong acid produces an ester from the alcohol and acid
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Mechanism of the Fischer Esterification
acid-catalyzed, nucleophilic acyl substitution of a carboxylic acid
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Mechanism of the Fischer Esterification
When 18O-labeled methanol reacts with benzoic acid, the methyl benzoate produced is 18O-labeled but the water produced is unlabeled
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Carboxylic Acids into Amides
Amides difficult to prepare. Carboxylic acids more easily react with amines to make salts. Reaction with DCC makes better leaving group that will be kicked out by an amine.
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Carboxylic Acids into Alcohols
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Carboxylic Acids into Alcohols
Preferred over LiAlH4 due to relative ease and safety. Selectively reduces Carboxylic acids in presence of nitros
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21.4 Chemistry of Acid Halides
Acid chlorides are prepared from carboxylic acids by reaction with SOCl2 Reaction of a carboxylic acid with PBr3 yields the acid bromide
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Reactions of Acid Halides
Nucleophilic acyl substitutions Halogen replaced by OH, by OR, or by NH2 Reduction yields a primary alcohol Grignard reagent yields a tertiary alcohol
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Hydrolysis: Acid Halides into Acids
Acid chlorides react with water to yield carboxylic acids HCl is generated during the hydrolysis: a base is added to remove the HCl
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Acid Halides to Esters Esters are produced in the reaction of acid chlorides with alcohols in the presence of pyridine or NaOH. This is called Alcoholysis The reaction is better with less steric bulk
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Aminolysis: Acid Halides into Amides
Amides result from the reaction of acid chlorides with NH3, primary (RNH2) and secondary amines (R2NH) HCl produced is neutralized by the amine or an added base The reaction with tertiary amines (R3N) gives an unstable species that cannot be isolated
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Reduction: Acid Chlorides into Alcohols
LiAlH4 reduces acid chlorides to yield aldehydes and then primary alcohols
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Acid Chlorides with Organometallic Reagents
Grignard reagents react with acid chlorides to yield tertiary alcohols in which two of the substituents are the same
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Formation of Ketones from Acid Chlorides
Acid chloride with a lithium diorganocopper (Gilman) reagent, Li+ R2Cu
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21.5 Chemistry of Acid Anhydrides
Prepared by nucleophilic acyl substitution of a carboxylate with an acid chloride
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Reactions of Acid Anhydrides
Similar to acid chlorides in reactivity
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Acetylation Acetic anhydride forms acetate esters from alcohols and N-substituted acetamides from amines
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21.6 Chemistry of Esters Many esters are pleasant-smelling liquids: fragrant odors of fruits and flowers Also present in fats and vegetable oils
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Preparation of Esters Esters are usually prepared from carboxylic acids
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Reactions of Esters Less reactive toward nucleophiles than are acid chlorides or anhydrides Cyclic esters are called lactones and react similarly to acyclic esters
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Hydrolysis: Esters into Carboxylic Acids
An ester is hydrolyzed by aqueous base or aqueous acid to yield a carboxylic acid plus an alcohol
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Mechanism of Ester Hydrolysis: Base catalysed
Hydroxide catalysis via an addition intermediate
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Mechanism of Ester Hydrolysis: Acid catalysed
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Aminolysis: Esters into Amides
Ammonia reacts with esters to form amides
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Reduction: Esters into Alcohols
Reaction with LiAlH4 yields primary alcohols
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Mechanism of Reduction of Esters
Hydride ion adds to the carbonyl group, followed by elimination of alkoxide ion to yield an aldehyde Reduction of the aldehyde gives the primary alcohol Use bulky DIBAH to stop at an aldehyde
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Reaction of Esters with Grignard Reagents
React with 2 equivalents of a Grignard reagent to yield a tertiary alcohol
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21.7 Chemistry of Amides Amides are abundant in all living organisms…proteins, nucleic acids, and other pharmaceuticals have amid functional groups
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Preparation of Amides Prepared by reaction of an acid chloride with ammonia, monosubstituted amines, or disubstituted amines
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Reactions of Amides Heating in either aqueous acid or aqueous base produces a carboxylic acid and amine Acidic hydrolysis by nucleophilic addition of water to the protonated amide, followed by loss of ammonia
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Basic Hydrolysis: Amides Acids
Addition of hydroxide and loss of amide ion
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Reduction: Amides into Amines
Reduced by LiAlH4 to an amine rather than an alcohol Converts C=O CH2
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Mechanism of Reduction
Addition of hydride to carbonyl group Loss of the oxygen as an aluminate anion to give an iminium ion intermediate which is reduced to the amine
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Uses of Reduction of Amides
Works with cyclic and acyclic Good route to cyclic amines
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21.8 Chemistry of Thioesters and Acyl Phosphates: Biological Carboxylic Acid Derivatives
Nucleophilic carboxyl substitution in nature often involves a thioester or acyl phosphate Acetyl CoA’s are most common thioesters in nature
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21.9 Polyamides and Polyesters: Step-Growth Polymers
Reactions occur in distinct linear steps, not as chain reactions Reaction of a diamine and a diacid chloride gives an ongoing cycle that produces a polyamide A diol with a diacid leads to a polyester
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Polyamides (Nylons) Heating a diamine with a diacid produces a polyamide called Nylon® Nylon 66® is from adipic acid and hexamethylene-diamine at 280°C
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Polyesters The polyester from dimethyl terephthalate and ethylene glycol is called Dacron® and Mylar® to make fibers
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Biodegradable Polymers
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21.10 Spectroscopy of Carboxylic Acid Derivatives
Infrared Spectroscopy Acid chlorides absorb near 1800 cm1 Acid anhydrides absorb at 1820 cm1 and also at cm1 Esters absorb at 1735 cm1, higher than aldehydes or ketones Amides absorb near the low end of the carbonyl region
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Nuclear Magnetic Resonance Spectroscopy
Hydrogens on the carbon next to a C=O are near 2 in the 1H NMR spectrum. All acid derivatives absorb in the same range so NMR does not distinguish them from each other
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13C NMR 13C NMR is useful for determining the presence or absence of a carbonyl group in a molecule of unknown structure Carbonyl carbon atoms of the various acid derivatives absorb from 160 to 180
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Which of the following is not an acyl derivative?
Learning Check: Which of the following is not an acyl derivative? 1. 2. 3. 4. 5. 1 2 3 4 5
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Which of the following is not an acyl derivative?
Solution: Which of the following is not an acyl derivative? 1. 2. 3. 4. 5. 1 2 3 4 5
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Which of the following is not an ester?
Learning Check: Which of the following is not an ester? 1. 2. 4. 3. 1 2 3 4
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Which of the following is not an ester?
Solution: Which of the following is not an ester? 1. 2. 4. 3. 1 2 3 4
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All lactams can be classified as amides and vice versa.
Learning Check: All lactams can be classified as amides and vice versa. True False
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All lactams can be classified as amides and vice versa.
Solution: All lactams can be classified as amides and vice versa. True False
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Learning Check: Name the following: ethyl-N-2-butylmethylamide
N-(2-methylbutyl)propanamide N-methylbutylpropane amide N-(2-methylbutyl)propane amide N-(2-methylbutylethyl) amide 3,4-dimethylphenyl pentanoate 3,4-dimethylpentyl benzenoate 3,4-dimethylpentyl benzoate benzyl 3,4-dimethylpentanoate phenyl 3,4-dimethylpentanoate
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Solution: Name the following: ethyl-N-2-butylmethylamide
N-(2-methylbutyl)propanamide N-methylbutylpropane amide N-(2-methylbutyl)propane amide N-(2-methylbutylethyl) amide 3,4-dimethylphenyl pentanoate 3,4-dimethylpentyl benzenoate 3,4-dimethylpentyl benzoate benzyl 3,4-dimethylpentanoate phenyl 3,4-dimethylpentanoate
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Learning Check: Name the following: 3-ethyl-3-phenylpropanoyl bromide
3-phenyl-3-ethylpropanoyl bromide 1-ethyl-1-phenylpropanoyl bromide 1-phenyl-1-ethylpropanoyl bromide 3-phenylpentanoyl bromide
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Solution: Name the following: 3-ethyl-3-phenylpropanoyl bromide
3-phenyl-3-ethylpropanoyl bromide 1-ethyl-1-phenylpropanoyl bromide 1-phenyl-1-ethylpropanoyl bromide 3-phenylpentanoyl bromide
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Learning Check: Which of the following is true concerning the given two carboxylic acid derivatives? Only molecule A can be hydrolyzed. Only molecule B can be hydrolyzed. Both molecules can be hydrolyzed, but A will react faster than B. Both molecules can be hydrolyzed, but B will react faster than A. A and B can be hydrolyzed at roughly the same rate.
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Solution: Which of the following is true concerning the given two carboxylic acid derivatives? Only molecule A can be hydrolyzed. Only molecule B can be hydrolyzed. Both molecules can be hydrolyzed, but A will react faster than B. Both molecules can be hydrolyzed, but B will react faster than A. A and B can be hydrolyzed at roughly the same rate.
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Determine the product of the following:
Learning Check: Determine the product of the following: 2. 1. 3. 4. 5. 1 2 3 4 5
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Determine the product of the following:
Solution: Determine the product of the following: 2. 1. 3. 4. 5. 1 2 3 4 5
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Which step occurs first in the mechanism of Fischer esterification?
Learning Check: Which step occurs first in the mechanism of Fischer esterification? attack of the nucleophile on the carbonyl carbon protonation of the carbonyl oxygen loss of water from the tetrahedral carbonyl addition intermediate protonation of the alcohol oxygen formation of the tetrahedral carbonyl addition intermediate
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Which step occurs first in the mechanism of Fischer esterification?
Solution: Which step occurs first in the mechanism of Fischer esterification? attack of the nucleophile on the carbonyl carbon protonation of the carbonyl oxygen loss of water from the tetrahedral carbonyl addition intermediate protonation of the alcohol oxygen formation of the tetrahedral carbonyl addition intermediate
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Learning Check: Which of the following is the tetrahedral intermediate that appears in the Fischer esterification of ethanol and benzoic acid? 2. 1. 3. 4. 5. 1 2 3 4 5
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Solution: Which of the following is the tetrahedral intermediate that appears in the Fischer esterification of ethanol and benzoic acid? 2. 1. 3. 4. 5. 1 2 3 4 5
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What is the expected product for the following reaction?
Learning Check: What is the expected product for the following reaction? 1. 2. 3. 5. 4. 1 2 3 4 5
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What is the expected product for the following reaction?
Solution: What is the expected product for the following reaction? 1. 2. 3. 5. 4. 1 2 3 4 5
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What sequence of reagents will accomplish the following?
Learning Check: What sequence of reagents will accomplish the following? 1. 2. 3. 4. 5. 1 2 3 4 5
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What sequence of reagents will accomplish the following?
Solution: What sequence of reagents will accomplish the following? 1. 2. 3. 4. 5. 1 2 3 4 5
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Predict the product of the following:
Learning Check: Predict the product of the following: 1. 2. 3. 4. none of these 5. 1 2 3 4 5
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Predict the product of the following:
Solution: Predict the product of the following: 1. 2. 3. 4. none of these 5. 1 2 3 4 5
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What sequence of reagents will accomplish the following?
Learning Check: What sequence of reagents will accomplish the following? 1. 2. 3. 4. 5. 1 2 3 4 5
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What sequence of reagents will accomplish the following?
Solution: What sequence of reagents will accomplish the following? 1. 2. 3. 4. 5. 1 2 3 4 5
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Predict the product of the following:
Learning Check: Predict the product of the following: 1. 2. 3. 5. 4. 1 2 3 4 5
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Predict the product of the following:
Solution: Predict the product of the following: 1. 2. 3. 5. 4. 1 2 3 4 5
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Learning Check: Identify the compound that will react fastest when heated in aqueous NaOH. 1. 2. 3. 4. 5. 1 2 3 4 5
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Solution: Identify the compound that will react fastest when heated in aqueous NaOH. 1. 2. 3. 4. 5. 1 2 3 4 5
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