aldehydes & ketones ichemistry3.chemistry.sc.chula.ac.th/images/download/bsac...nomenclature...
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2302272 – Org Chem II – Part I
Lecture 3
Aldehydes & Ketones I
Instructor: Dr. Tanatorn Khotavivattana
E-mail: [email protected]
Recommended Textbook:
Chapter 18 in Organic Chemistry, 8th Edition, L. G. Wade, Jr., 2010,
Prentice Hall (Pearson Education)
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Aldehyde = Latin “alcohol dehydrogenatus” (dehydrogenated alcohol)1
Ketone = “Aketon” (an old German word for acetone)
formaldehyde
methanal
acetone
propanone
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Chapter 18 – Wade - Prentice Hall
Compounds containing a carbonyl group 2
• Important in chemistry, biochemistry, biology
• Constituents of fabrics, flavourings, plastics, drugs, etc.
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Chapter 18 – Wade - Prentice Hall
Aldehydes and Ketones 3
• Ketones and aldehydes are similar in structure, and they have similar properties
• In most cases, aldehydes are more reactive than ketones
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Chapter 18 – Wade - Prentice Hall
Structure of the Carbonyl Group 4
• sp2 hybridised carbon
• Bonded to 3 other atoms through coplanar sigma bonds oriented about 120° apart
• Unhybridized p orbital overlaps with a p orbital of Oxygen to form a pi bond
• C=O bond has similar geometry to C=C but C=O bond is shorter, stronger and
more polarized than C=C bond
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Chapter 18 – Wade - Prentice Hall
Structure of the Carbonyl Group 5
• C=O has a large dipole moment because O is more electronegative than C and
the bonding electrons are not shared equally (resonance)
more bonds and less
charge separation
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Chapter 18 – Wade - Prentice Hall
Nomenclature – IUPAC Names of Aldehydes 6
• Replace -e of alkane name with –al
• Aldehyde C is at the end of a chain: (almost) always number 1
• If aldehyde is attached to a large unit (ring): use suffix carbaldehyde
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Chapter 18 – Wade - Prentice Hall
Nomenclature – IUPAC Names of Ketones 7
• Replace -e of alkane name with –one
• Number longest chain containing C=O from the end closest to C=O
• Indicate position of the C=O by a number
• In cyclic ketones, carbonyl carbon is assigned number 1
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Chapter 18 – Wade - Prentice Hall
Nomenclature – Aldehydes and Ketones 8
• Also be named as a substituent on a molecule with a higher priority group
• A ketone or aldehyde carbonyl is named by the prefix
oxo- if it is included as part of the longest chain in the
root name
Priority Ranking
• When an aldehyde group is a substituent and not part
of the longest chain, it is named by the prefix formyl
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Nomenclature – Common Names of Aldehydes 9
• Derived from the common names of carboxylic acids
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Nomenclature – Common Names of Ketones 10
• Naming 2 alkyl groups bonded to the C=O; add ketone at the end
• Some ketones have historical common names
Chapter 18 – Wade - Prentice Hall
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Problem #111
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Physical Properties of Aldehydes and Ketones 12
• Polarization of the carbonyl group creates dipole–dipole attractions between
the molecules
Chapter 18 – Wade - Prentice Hall
• Higher boiling points than for hydrocarbons and ethers of similar M.W.
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Physical Properties of Aldehydes and Ketones 13
• Pure aldehydes or ketones cannot form H bonding with each other
Chapter 18 – Wade - Prentice Hall
• They can form H bonding with compounds having O-H or N-H bonds
• Aldehydes and ketones are good solvents for polar hydroxylic compounds
(eg. Alcohols)
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Physical Properties of Aldehydes and Ketones
Chapter 18 – Wade - Prentice Hall
14
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Chapter 18 – Wade - Prentice Hall
15Industrial of Aldehydes and Ketones
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16Reactions of Aldehydes and Ketones
Chapter 18 – Wade - Prentice Hall
• The most common reaction is nucleophilic addition (addition of a nucleophile and
a proton across the double bond)
• The reactivity of the carbonyl group arises from the electronegativity of the
oxygen atom and the resulting polarization of the carbon–oxygen double bond
• The electrophilic carbonyl carbon atom is sp2 hybridized and flat, leaving it
relatively unhindered and open to attack from either face of the double bond
• The carbon atom changes hybridization from sp2 to sp3
• The electrons of the pi bond are forced out to the oxygen atom to form an alkoxide
anion, which protonates to give the product of nucleophilic addition
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17Reactions of Aldehydes and Ketones
Aldehydes vs. Ketones towards Nucleophilic Addition
• In most cases, aldehydes are more reactive than ketones; they usually react more
quickly (Kinetics), and the position of the equilibrium usually lies more toward the
products (Thermodynamics) than with ketones
Steric Effect
Chapter 18 – Wade - Prentice Hall
Electronic Effect
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Chapter 10 – Wade - Prentice Hall
18Nucleophile: “Alkyl” or “Aryl” source
• Formula: R—Mg—X (alkyl magnesium halide)
• Reacts like R- +MgX
• May be formed from any halides (alkyl, vinyl or aryl halides)
• Ethers are used as solvents to stabilise the complex
Grignard Reagents
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Chapter 10 – Wade - Prentice Hall
19
• Formula: R—Li (alkyl lithium)
• Reacts like R- +Li
• May be formed from any halides (alkyl, vinyl or aryl halides)
• Ether not necessary, wide variety of solvents can be used
Organolithium Reagents
Examples
Nucleophile: “Alkyl” or “Aryl” source
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Chapter 10 – Wade - Prentice Hall
20
#1: Sodium Borohydride (NaBH4) #2: Lithium Aluminium Hydride (LiAlH4)
Common Reagents
• Aluminium is less electronegative than boron. Therefore, lithium aluminium hydride(LAH) is amuch stronger reducing agent, and it is much more difficult to work with.
• LAH reacts explosively with water and alcohols, liberating hydrogen gas and sometimesstarting fires. Sodium borohydride reacts slowly with water and alcohols.
• Sodium borohydride is a convenient and highly selective reducing agent.
Nucleophile: “Hydride” source
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21Reactions of Aldehydes and Ketones
Chapter 18 – Wade - Prentice Hall
1) Reaction with Grignard Reagent (and other carbanions, R-)
2) Hydride Reductions
• Attack by R- gives an alkoxide that protonates to form an alcohol
• Attack by hydride gives an alkoxide that protonates to form an alcohol
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Problem #222
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23Reactions of Aldehydes and Ketones
3) Formation of Cyanohydrins
• Hydrogen cyanide (HCN) is a toxic, water-soluble liquid that boils at 26 °C
Chapter 18 – Wade - Prentice Hall
• The conjugate base is the cyanide ion (CN-), which is a strong nucleophile;
It attacks ketones and aldehydes to give addition products called cyanohydrins
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24Reactions of Aldehydes and Ketones
3) Formation of Cyanohydrins
Chapter 18 – Wade - Prentice Hall
• The millipede stores mandelonitrile which
is a cyanohydrin of benzaldehyde
• Cyanohydrin formation is reversible; when attacked, it discharges
mandelonitrile through a reaction chamber containing enzymes that
catalyse the conversion of the cyanohydrin to benzaldehyde and HCN
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• Aldehydes are more likely than ketones to form stable cyanohydrin (electronic and
steric effects); Formaldehyde is even more reactive than other aldehydes.
253) Formation of Cyanohydrins
Chapter 18 – Wade - Prentice Hall
• Cyanohydrin formation is reversible; the equilibrium constant may or may not
favour the cyanohydrin
< <Reactivity
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26Reactions of Aldehydes and Ketones
4) Hydration
• In an aqueous media, a ketone or an aldehyde is in equilibrium with a geminal diol
Chapter 18 – Wade - Prentice Hall
• The reaction is very slow because water is a weak nucleophile; Either activation
of the electrophile (the carbonyl group) or of the nucleophile (the water) is required
• A carbonyl group that is protonated (or bonded to some other electrophile) is
strongly electrophilic, inviting attack by a weak nucleophile
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Chapter 18 – Wade - Prentice Hall
• Mechanism for acid-catalysed hydration
Reactions of Aldehydes and Ketones
4) Hydration
• Mechanism for basic-catalysed hydration
Hydroxide is a much stronger nucleophile than water
27
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4) Hydration28
• With most ketones, the equilibrium favours the unhydrated keto form
Chapter 18 – Wade - Prentice Hall
• The reactivity follows the same trend as other reactions
Ketone
Aldehyde
Formaldehyde
Reactivity
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29Reactions of Aldehydes and Ketones
5) Formation of Acetals
• Aldehydes and Ketones react with Alcohols to form Acetals
• Acetal formation must be acid-catalysed (not base-catalysed)
Chapter 18 – Wade - Prentice Hallhemi = half
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Example
30
Chapter 18 – Wade - Prentice Hall
5) Formation of Acetals
Mechanism:
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31
Chapter 18 – Wade - Prentice Hall
5) Formation of Acetals
Mechanism (continued):
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32
Chapter 18 – Wade - Prentice Hall
5) Formation of Acetals
Carbohydrate Chemistry
• Glucose is a six-carbon
sugar that is most stable
as a hemiacetal
Alcohol
Aldehyde
2 x
Alcohol
• Lactose is a disaccharide (composed of two sugar units) that has one acetal and
one hemiacetal
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33
Chapter 18 – Wade - Prentice Hall
5) Formation of Acetals
Equilibrium of Acetal Formation
• Acetal formation is reversible
• For simple aldehydes, equil. const. generally favour the acetals
• With hindered aldehydes and most ketones, equil. const. favour the carbonyl
• Most acetals are hydrolysed by shaking with dilute acid in water
• Large excess of water drives acetals back to C=O
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Problem #334
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35
Chapter 18 – Wade - Prentice Hall
5) Formation of Acetals
Cyclic Acetals
• Formation of an acetal using a diol as the alcohol gives a cyclic acetal
• Cyclic acetals often have more favourable equilibrium constants, since there is a
smaller entropy loss (2 molecules condense instead of 3 for normal alcohol)
• Ethylene glycol is often used to make cyclic acetals
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365) Formation of Acetals
Acetals as Protecting Groups
• If the aldehyde is protected as an acetal, it is unreactive toward a Grignard reagent
• Acetals are stable to strong bases and nucleophiles
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Problem #437
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38Reactions of Aldehydes and Ketones
6) Formation of Imines
• Ammonia or a primary amine reacts with ketone or aldehyde to form an imine via
condensation reaction
Chapter 18 – Wade - Prentice Hall
• Imines are nitrogen analogues of aldehydes and ketones with C=N bond in place
of C=O bond
• Like amines, imines are basic; a substituted imine is also called a Schiff base
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396) Formation of Imines
Chapter 18 – Wade - Prentice Hall
Mechanism – acid-catalysed
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406) Formation of Imines
Chapter 18 – Wade - Prentice Hall
• The proper pH is crucial to imine formation
• The second half of the mechanism is acid-catalysed, so the solution must be
somewhat acidic.
• However, if the solution is too acidic, the amine becomes protonated and
non-nucleophilic, inhibiting the first step.
Example
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416) Formation of Imines
Chapter 18 – Wade - Prentice Hall
Other Types of Amines
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426) Formation of Imines
Chapter 18 – Wade - Prentice Hall
Reaction with 2,4-DNP – a qualitative test for aldehydes and ketones
2,4-dinitrophenylhydrazine
(2,4-DNP)dinitrophenylhydrazone
yellow, orange or red precipitate
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43
Chapter 18 – Wade - Prentice Hall
Reactions of Aldehydes and Ketones
7) Oxidation of Aldehydes
• Easily oxidized to carboxylic acids by common oxidants (unlike ketones)
• Common oxidants: bleach (sodium hypochlorite), chromic acid, permanganate
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Chapter 18 – Wade - Prentice Hall
Reactions of Aldehydes and Ketones
7) Oxidation of Aldehydes – Tollens Test
• Tollens reagent: a solution of silver–ammonia complex
• Silver ion (Ag+) is a mild oxidising agent; oxidises aldehydes selectively
• Convenient functional-group test for aldehydes
• If an aldehyde is present, its oxidation reduces
silver ion to metallic silver in the form of a black
suspension or a silver mirror deposited on the
inside of the container.
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Problem #545
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Chapter 18 – Wade - Prentice Hall
Reactions of Aldehydes and Ketones
8) Reduction of Aldehydes and Ketones
• Most commonly reduced by sodium borohydride (NaBH4)
• Lithium aluminum hydride (LiAlH4) also works, but it is more powerful
(less selective), and it is much more difficult to work with
• Sodium triacetoxyborohydride [NaBH(OAc)3] is less reactive than NaBH4,
and it selectively reduces aldehydes even in the presence of ketones.
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Chapter 18 – Wade - Prentice Hall
Reactions of Aldehydes and Ketones
8) Reduction of Aldehydes and Ketones
• Clemmensen Reduction: convert aldehydes and ketones to alkanes
• The carbonyl compound is heated with an excess of amalgamated zinc and
hydrochloric
• Limitation: some compounds might decompose in hot and acidic conditions
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Chapter 18 – Wade - Prentice Hall
Reactions of Aldehydes and Ketones
8) Reduction of Aldehydes and Ketones
• Wolff–Kishner Reduction: convert aldehydes and ketones to alkanes
• The carbonyl compounds is treated with hydrazine to form hydrazones, which is
heated with a strong base such as KOH to facilitate the elimination of N2 gas
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Chapter 18 – Wade - Prentice Hall
Reactions of Aldehydes and Ketones
9) Wittig Reaction
• Converts C=O to a new C=C using phosphorus ylide (phosphorus-stabilized
carbanion)
• Wittig received the Nobel Prize in Chemistry in 1979 for this discovery
• Often results in a mixture of cis- and trans- isomers
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Chapter 18 – Wade - Prentice Hall
9) Wittig Reaction
Preparation of Phosphorus Ylide
• Step 1: nucleophilic attack by triphenylphosphine on an unhindered alkyl halide
to give alkyltriphenylphosphonium salt
• Step 2: The phosphonium salt is treated with a strong base (usually butyllithium)
to abstract a proton
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519) Wittig Reaction – Mechanism
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Problem #652
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Homework – 1 53
Chapter 18 – Wade - Prentice Hall
Homework – 2
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Chapter 18 – Wade - Prentice Hall
Homework – 3
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Chapter 18 – Wade - Prentice Hall
Homework – 4