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Aldehydes and Ketones | DAT Hydrogen Bonding • Aldehydes and ketones don’t hydrogen bond with themselves or each other Boiling Points of Different Functional Groups Aldehydes’ and ketones’ polarity gives them higher boiling points than alkanes and alkenes. However, their inability to hydrogen bond makes their boiling points lower than alcohols. • Aldehydes and Ketones Won’t Hydrogen Bond Together Aldehydes and ketones don’t hydrogen bond because alkyl hydrogens don’t participate in hydrogen bonds. • Like alcohols, they are water soluble o But not if their non-polar alkyl chains are too long Aldehyde and Ketone Behavior • Aldehydes and ketones behave in either of 2 ways during reactions • First, as we’ve seen, they can behave as substrates o The target of nucleophilic attack Aldehydes and ketones will, however, hydrogen bond with molecules that hydrogen bond o Like alcohols Alcohols Do Hydrogen Bond with Aldehydes and Ketones Boiling Point & Solubility • Aldehydes and ketones are polar o Dipole-dipole attractions make their boiling points higher than that of alkanes and alkenes • But they cannot hydrogen bond o So their boiling points are less than alcohols Ketone Behaving as a Substrate for Nucleophilic Attack As we’ve previously seen, aldehydes and ketones can serve as the substrates of nucleophilic attack. The entering nucleophile attacks the carbonyl carbon, sending the C=O pi electrons up to the oxygen. • Alternatively, they can also behave as acids by donating a hydrogen o But what hydrogen? 1 © 2017 J Co Review, Inc., Accessed by Guest on 06-14-2017 Aldehydes and Ketones | DAT • • All the hydrogens on the molecule are alkyl hydrogens o From what we’ve learned thus far, alkyl hydrogens are not acidic Before we talk about which hydrogens are acidic, let’s address a common notation o Carbons are labeled using Greek letters o Carbons that are 1 bond away from the carbonyl are alpha (α) o Carbons 2 bonds from the carbonyl are beta (β), etc. o Hydrogens are given the letter of the carbon to which they are bonded Labeling Carbons In carbonyl-containing molecules, carbons adjacent to the carbonyl are dubbed alpha. Carbons next to those are beta, followed by gamma, delta, etc. Hydrogens are referenced using the Greek letter on their carbon. So, for example, the hydrogens on a beta carbon are all beta hydrogens. • • It turns out that one alpha hydrogen will be acidic To understand why, consider what happens when an alpha hydrogen is donated o A lone pair is left on the carbon o That lone pair can drop into a C=C double bond Thus pushing the C=O pi electrons up to the oxygen Donating an α-Hydrogen When the α-hydrogen is donated, it leaves a lone pair on the α-carbon. That lone pair can form a C=C double bond, thereby pushing the C=O pi electrons up to the oxygen. • In other words, we have resonance o And resonance stabilizes the molecule The Deprotonated Ketone has Resonance The ketone was willing to lose an α-hydrogen because doing so allowed the molecule to form a resonance structure, and resonance stabilizes a molecule. • • The second of the two resonance structures shown above is known as an enolate ion o En- because it’s an alkene o –ol- because the C-O bond would form an alcohol, if protonated o –ate basically tells us the alcohol is deprotonated Once the ketone has donated its α-hydrogen, it can behave as a nucleophile o That is, it can attack an electrophile to form a bond 2 © 2017 J Co Review, Inc., Accessed by Guest on 06-14-2017 Aldehydes and Ketones | DAT Deprotonated Ketone Acting as a Nucleophile Here, the ketone acted as an acid and donated an α-hydrogen. Its lone pair allows the molecule to behave as a nucleophile by attacking an electrophile. Dicarbonyls • An α-hydrogen is extremely acidic if it is flanked by 2 carbonyls o The molecule is even more stabilized by resonance Resonance Structures of the Deprotonated Dicarbonyl When an α-hydrogen between the carbonyls is donated, we form three resonance structures. The red arrows on each structure show the electron path that produces the following structure. The red arrows on the third structure indicate the electron path that produces the first structure. • α-Hydrogens between Two Carbonyls When flanked by two carbonyls, an α-hydrogen is very acidic. Loss of either (but not both) of these hydrogens will produce three resonance structures, pictured below. Remember, more resonance = more stability. • • Before, losing an α-hydrogen gave us two resonance structures Losing an α-hydrogen flanked by two carbonyls will actually give us three o So the deprotonated molecule will be more stable o And so it won’t “mind” losing that proton If put in acid, the structure can be protonated to form an alcohol o The alcohol will also be stabilized via hydrogen bonding Hydrogen Bonding of the Alcohol If the dicarbonyl loses a flanked α-hydrogen, the molecule can then be protonated by an acid. Hydrogen bonding between the alcoholic hydrogen and the carbonyl aids in stabilizing the molecule. 3 © 2017 J Co Review, Inc., Accessed by Guest on 06-14-2017 Aldehydes and Ketones | DAT Keto-Enol Tautomerization • Ketones (not deprotonated ketones) undergo a spontaneous reaction o An α-hydrogen moves from the α-carbon to the carbonyl o This forms an enol (a) (b) Keto-Enol Tautomerization The ketone in (a) undergoes a spontaneous reaction, whereby it loses an alpha proton to the carbonyl, forming an alkene and an alcohol- (b) an enol. The reaction will occur rapidly back and forth under standard conditions. • Note that this is not resonance; it’s a reaction o Resonance structures involve only the movement of electrons o Here, we are actually moving an atom, the H+ Important Aldehydes and Ketones • Know the most common aldehyde and ketone: Acidity of Aldehydes vs. Ketones • Suppose an aldehyde and ketone each lose an α-hydrogen (a) (b) Ketones and Aldehydes as Acids The (a) ketone and (b) aldehyde have lost an αhydrogen. In the resulting molecule, the ketone has an alkyl group, while the aldehyde has a hydrogen. Alkyl groups are electron donors, and so it tries to push more electrons to the negative carbanion (red arrow). This makes the molecule less stable than the deprotonated aldehyde, in which the hydrogen does not behave as an electron donor. • • • The alkyl groups of ketones are electron donors o So they try to put more negative charge on the carbanion • Not stable The hydrogen of the aldehyde is not an electron donor o So it doesn’t force more negative charge to the carbanion Since the carbanion is more stable on the aldehyde than the ketone, the aldehyde is more willing to release that α-proton o That is, the aldehyde is more acidic Formaldehyde and Acetone The most common aldehyde you will see is formaldehyde, and the most common ketone you will come across is acetone. 4 © 2017 J Co Review, Inc., Accessed by Guest on 06-14-2017