21.5 Reactions of Carboxylic Acids

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In this chapter, we focus on four classes of organic compounds derived from carboxylic acids. Under the general formula of each is a drawing to show how.
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Presentation transcript:

21.5 Reactions of Carboxylic Acids LiAlH4 is a strong reducing agent that can convert an acid to a primary alcohol The LAH acts as a base first Then, an aldehyde is produced Copyright 2012 John Wiley & Sons, Inc.

21.5 Reactions of Carboxylic Acids LiAlH4 is a strong reducing agent that can convert an acid to a primary alcohol The aldehyde is further reduced to the alcohol Can the reduction be stopped at the aldehyde? Copyright 2012 John Wiley & Sons, Inc.

21.5 Reactions of Carboxylic Acids The more mild borane reagent can also be used to promote the reduction Reduction with borane is selective compared to LAH reduction Practice with conceptual checkpoint 21.11 Copyright 2012 John Wiley & Sons, Inc.

21.6 Introduction to Carboxylic Acid Derivatives The reduction of acids with LAH or borane result in a decrease in the oxidation number for carbon. HOW? There are also many reactions where carboxylic acids don’t change the oxidation state What criteria must Z fulfill so that there is no change in the oxidation state Copyright 2012 John Wiley & Sons, Inc.

21.6 Introduction to Carboxylic Acid Derivatives When Z is a heteroatom, the compound is called a carboxylic acid derivative Because it has the same oxidation state, a nitrile is also an acid derivative despite not having a carbonyl group Copyright 2012 John Wiley & Sons, Inc.

21.6 Introduction to Carboxylic Acid Derivatives Acid halides and anhydrides are relatively unstable, so they are not common in nature – we will discuss their instability in detail later in this chapter Some naturally occurring esters are known to have pleasant odors Copyright 2012 John Wiley & Sons, Inc.

21.6 Introduction to Carboxylic Acid Derivatives Amides are VERY common in nature What type of molecule in nature includes amide linkages? Many other compounds feature amides including some natural sedatives like melatonin Copyright 2012 John Wiley & Sons, Inc.

21.6 Introduction to Carboxylic Acid Derivatives To name an acid halide, replace “ic acid” with “yl halide” Copyright 2012 John Wiley & Sons, Inc.

21.6 Introduction to Carboxylic Acid Derivatives Alternatively, the suffix, “carboxylic acid” can be replaced with “carbonyl halide” Copyright 2012 John Wiley & Sons, Inc.

21.6 Introduction to Carboxylic Acid Derivatives Acid anhydrides are named by replacing “acid” with “anhydride” Copyright 2012 John Wiley & Sons, Inc.

21.6 Introduction to Carboxylic Acid Derivatives Asymmetrical acid anhydrides are named by listing the acids alphabetically and adding the word anhydride Copyright 2012 John Wiley & Sons, Inc.

21.6 Introduction to Carboxylic Acid Derivatives Esters are named by naming the alkyl group attached to the oxygen followed by the carboxylic acid’s name with the suffix “ate” Copyright 2012 John Wiley & Sons, Inc.

21.6 Introduction to Carboxylic Acid Derivatives Amides are named by replacing the suffix “ic acid” or “oic acid” with “amide” Copyright 2012 John Wiley & Sons, Inc.

21.6 Introduction to Carboxylic Acid Derivatives If the nitrogen atom of the amide group bears alkyl substituents, their names are placed at the beginning of the name with N as their locant Copyright 2012 John Wiley & Sons, Inc.

21.6 Introduction to Carboxylic Acid Derivatives Nitriles are named by replacing the suffix “ic acid” or “oic acid” with “onitrile” Practice with conceptual checkpoints 21.12 and 21.13 Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Carboxylic acid derivatives have electrophilic sites Where? Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Reactivity can be affected by Induction Resonance Sterics Quality of leaving group Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Let’s examine the acid chloride The electronegative chlorine enhances the electrophilic character of the carbonyl. HOW? There are 3 resonance contributors to the acid chloride The chlorine does not significantly donate electron density to the carbonyl. HOW does that affect its quality as an electrophile Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Let’s examine the acid chloride Describe how the presence of the chloride affects the sterics of the nucleophilic attack on the carbonyl The chloride is a good leaving group, which also enhances its reactivity Considering all of the factors involved, the acid chloride is quite reactive Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Amides are the least reactive acid derivative Examine the factors below to explain amide reactivity Induction Resonance Sterics Quality of leaving group Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Aldehydes and ketones are also electrophilic, but they do not undergo substitution WHY? Consider induction, resonance, sterics, and quality of leaving group Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Nucleophilic acyl substitution is a two-step process Because C=O double bonds are quite stable, the “loss of leaving group” step should occur if a leaving group is present – H and –R do not qualify as leaving groups. WHY? Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Let’s analyze a specific example The highest quality leaving group leaves the tetrahedral intermediate Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Do NOT draw the acyl substitution with an SN2 mechanism Sometimes a proton transfer will be necessary in the mechanism Under acidic conditions, (-) charges rarely form. WHY? Under basic conditions, (+) charges rarely form. WHY? Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Under acidic conditions, (-) charges rarely form The first step will NOT be nucleophilic attack The electrophile and nucleophile are both low in energy Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives H3O+ is unstable and drives the equilibrium forward by starting the reaction mechanism Now that the electrophile carries a (+) charge, it is much less stable (higher in energy. Complete the rest of the mechanism Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Under basic conditions, (+) charges rarely form The OH- is the most unstable species in the reaction and drives the equilibrium forward Continue the rest of the mechanism Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Neutral nucleophiles are generally less reactive, but they can still react if given enough time An intermediate with both (+) and (-) charge forms Intermediates with two (+) or two (-) charges are very unlikely to form. WHY? Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Depending on reaction conditions, up to 3 proton transfers may be necessary in the mechanism Draw a complete mechanism for the reaction below Will the reaction be reversible? What conditions could be employed to favor products? Practice with SkillBuilder 21.1 Copyright 2012 John Wiley & Sons, Inc.

21.7 Reactivity of Carboxylic Acid Derivatives Give necessary reaction conditions and a complete mechanism for the reaction below Describe how conditions could be modified to favor the products as much as possible Copyright 2012 John Wiley & Sons, Inc.

Study Guide for Sections 21.5-21.7 DAY 20, Terms to know: Sections 21.5-21.7 carboxylic acid derivitives, ester, anhydride, amide, nitrile, acid halide DAY 20, Specific outcomes and skills that may be tested on exam 3: Sections 21.5-21.7 Given reactants, be able to predict products and give complete mechanisms for reductions of carboxylic acids with LAH or BH3 Given a carboxylic acid precursor, be able to give sets of reagents and reaction conditions that could yield a given primary alcohol Be able to give IUPAC names for simple carboxylic acid derivatives Given the name of a carboxylic acid derivative, be able to draw its structure Be able to rank and explain why aldehydes, ketones, acids, and acid derivatives have varying strengths as electrophiles in substitution reactions including sterics, quality of leaving group, and SCARIO arguments Be able to describe in detail kinetic and thermodynamic analyses for reactions of carboxylic acid derivatives Given reactants, be able to predict products and give complete mechanisms for nucleophilic substitution reactions on acids and acid derivatives Given a precursor, be able to give sets of reagents and reaction conditions that could yield a given carboxylic acid derivative Given a carboxylic acid derivative, be able to predict products and give complete mechanisms for any of the reactions we discussed that acid derivatives undergo

Extra Practice Problems for Sections 21.5-21.7 Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period. 21.11 21.12 21.13 21.14 21.15 21.16 21.17 21.45 21.48

Day 21: EXAM 3 Prep for Day 22 Must Watch videos: https://www.youtube.com/watch?v=PwtW4KYLD9I (acid chloride formation and reaction, FLC) https://www.youtube.com/watch?v=EmkLnHK6UsI (esters, FLC) https://www.youtube.com/watch?v=XZsPXwl1COg (interconversions between derivitives, Moore) Other helpful videos: https://www.youtube.com/watch?v=PgtdNYbIwmY (ester hydrolysis, Moore) https://www.youtube.com/watch?v=e_hjQZs3DtA (anhydride reactions, AK lectures) https://www.youtube.com/watch?v=T7qI2hBmRTE (reactions with acid derivatives, UC-Irvine) https://www.youtube.com/watch?v=C29epYs9_zM (more on reactions with acid derivatives, UC-Irvine) http://ocw.uci.edu/lectures/chem_51c_organic_chemistry_lec_11_reaction_of_esters.html (reactions with esters, UC-Irvine) Read Sections 21.8-21.11