Aldehydes

1. Aldehydes Lecture Presented by: Victor R. Oribe P resented to: Dr. Leonisa O. Bernardo

2. Aldehydes contain carbonyl functional group. A carbonyl group is a carbon atom double bonded to an oxygen atom. The structural representation for a carbonyl group is:

3. Carbon-oxygen and carbon-carbon double bond differ in a major way. A carbon-oxygen bond is polar and a carbon-carbon double bond is nonpolar. The electronegativity of oxygen (3.5) is much greater than that of carbon (2.5). Hence the carbon-oxygen double bond is polarized, the oxygen atom acquiring a fractional negative charge and the carbon atom acquiring a fractional positive charge.

4. All carbonyl groups have a trigonal planar structure. The bond angles between the three atoms attached to the carbonyl carbon atom are 120 , as would be predicted using VSEPR theory.

5. An Aldehyde is a carbonyl-containing organic compound in which the carbonyl carbon atom has at least one hydrogen atom directly attached to it. The remaining group attached to the carbonyl carbon atom can be hydrogen, an alkyl group, a cycloalkyl group, or an aryl group ( Ar ).

6. In interpreting general condensed functional group structure such as RCHO, remember that carbon always has four bonds and hydrogen always has only one . H In RCHO, one of carbon’s bonds goes to the R group and one to H; t herefore, two bonds must go to O.

7. Linear notation for an aldehyde functional group and for an aldehyde itself are –CHO and RCHO respectively. Note that the ordering of the symbol H and O in these notations is HO, not OH (which denotes a hydroxyl group) In an aldehyde, the carbonyl group is always located at the end of a hydrocarbon chain. CH 3 – CH 2 – CH 2 – CH 2 – C - H O ║

8. An aldehyde functional group can be bonded to only one carbon atom because three of the four bonds from an aldehyde carbonyl carbon must go to oxygen and hydrogen. Thus, an aldehyde functional group is always found at the end of the carbon chain. Cyclic aldehydes are not possible. For an aldehyde carbonyl carbon atom to be part of a ring system it would have to form two bonds to ring atoms, which would give it five bonds.

9. Aldehydes are related to alcohol in some manner that alkenes are related to alkanes. Removal of hydrogen atoms from each of two adjacent carbon atoms in an alkane produces an alkene. In a like manner, removal of a hydrogen atom from the –OH group of an alcohol and from the carbon atom to which the hydroxyl group is attached produces a carbonyl group. C H O H -2H C ║ O Alcohol Aldehyde

10. Physical Properties of Aldehydes The C 1 and C 2 aldehydes are gases at room temperature. The C 3 through C 11 straight-chain saturated aldehydes are liquids, and the higher aldehydes are solid. The presence of alkyl groups tends to lower both boiling points and melting points, as does the presence of unsaturated in the carbon chain. The boiling point of aldehydes are intermediate between those of alcohols and alkanes of similar molecular mass.

11. Aldehydes have higher boiling points than alkanes because of dipole-dipole attractions between molecules. Carbonyl group polarity makes these dipole-dipole interactions possible. ║ ║ O C C O Unbranched Aldehydes C 1 C 3 C 5 C 7 C 2 C 4 C 6 C 8 A physical state summary for unbranched aldehydes at room temperature and pressure gas liquid Dipole-dipole attraction

12. Aldehydes have lower boiling points than the corresponding alcohols because no hydrogen bonding occurs as it does with alcohols. Water molecule can hydrogen-bond with aldehyde molecule. Aldehyde-water hydrogen bonding C O R H O O H H H H

13. This hydrogen bonding causes low-molecular-mass aldehydes to be water soluble. As the hydrocarbon portion get larger, the water solubility of aldehydes decreases. Low-molecular –mass aldehyde have pungent, penetrating, unpleasant odors, higher-molecular-mass aldehydes (above C 8 ) are more fragrant, especially benzaldehyde derivatives.

14. Naturally Occurring Aldehydes Aldehydes occurs widely in nature. Naturally occurring aldehydes are of higher molecular masses, usually have pleasant odor and flavors and are often used for these properties in consumer products (perfume, air fresheners, and the like). Vanillin Vanilla flavoring Benzaldehyde almod flavoring Cinnamaldehyde cinnamon flavoring

15. Nomenclature for Aldehyde The IUPAC rules for naming aldehydes: 1. Select as the parent carbon chain the longest chain that includes the carbon atom of the carbonyl group. CH 3 – CH 2 - CH 2 – CH – C – H ║ O 1 methane 2 ethane 3 propane 4 butane 5 pentane 5 4 3 2 1 Pentane Parent chain CH 2 CH 2

16. 2. Name the parent chain by changing the –e ending of the corresponding alkane to –al. Pentan e al 3. Determine the identity and location of any substituents, and append this information to the front of the parent chain name. CH 3 – CH 2 - CH 2 – CH – C – H ║ O CH 2 CH 3 1 methyl 2 ethyl 3 propyl 4 butyl 5 pentyl 2 1 ethyl 1 2 3 4 5 2 2- pentanal

17. H ║ 4 3 2 1 1 methyl 2 ethane 3 propane 4 butane 5 pentane butane butanal methyl 3 3- 3-methylbutanal

18. CH 3 – CH 2 – CH – CH 2 – C - H ║ OH O 5 4 3 2 1 Hydroxyl aldehyde

19. Assign IUPAC names to the following aldehydes 1) CH 3 – CH – C- H ║ O CH 3 2-Methylpropanal 2 ) CH 3 – CH – CH- C - H ║ O Cl Cl Dichlorobutanal

20. 3) CH 3 – CH 2 – CH- C - H ║ O CH 3 – CH 2 – CH 2 2-Ethylpentanal 4) H ║ O 2-Methylbutanal

21. The common name for simple aldehyde illustrate a second method for counting from one to four: form - , acet - , propion - , and butyr - The common names for aldehydes are one word rather than two or more. H – C - H ║ O Formaldehyde CH 3 – C - H ║ O Acetaldehyde CH 3 – CH 2 – C - H ║ O Propionaldehyde

22. IUPAC system for Naming Aromatic A ldehydes Aromatic aldehydes are names as derivatives of benzaldehyde , the parent compound. C - H ║ O Cl C - H ║ O CH 3 C - H O ║ O H Benzaldehyde 3-chloro-5-methylbenzaldehyde 4-hydroxybenzaldehyde

23. Preparation of Aldehydes Aldehydes can be produced by the oxidation of primary alcohol, using mild oxidizing agents such as KMnO 4 or K 2 Cr 2 O 7 . R – C – H ║ OH H oxidation R – C – H O Primary alcohol Aldehyde When this reaction is used for aldehyde preparation, reaction conditions must be sufficiently mild to avoid further oxidation of the aldehyde to a carboxylic acid.

24. The term Aldehyde stems from alcohol dehydrogenation , indicating that aldehydes are related to alcohols by the loss of hydrogen.

25. Predicting Products in Alcohol Oxidation Reaction Draw the aldehyde formed from the oxidation of each of the following alcohols. Assume that reaction conditions are sufficiently mild that any aldehydes produced are not oxidized further. 1) CH 3 – CH 2 – CH 2 - OH CH 3 – CH 2 – C – H ║ O 2) CH 3 – CH – CH 2 - OH CH 3 CH 3 – CH – C - H ║ O CH 3 3 ) CH 3 – C – CH 2 - OH CH 3 CH 3 CH 3 – C – C - H CH 3 CH 3 ║ O

26. 3) CH 3 - CH 2 – CH 2 – CH 2 – CH 2 - OH CH 3 - CH 2 – CH 2 – CH 2 – C - H ║ O 4 ) CH 3 - C – CH 2 – CH 2 – OH CH 3 CH 3 CH 3 - C – CH 2 – C – H CH 3 CH 3 ║ O 5) CH 3 – CH 2 – CH – CH 2 - OH CH 3 CH 3 – CH 2 – CH – C – H CH 3 ║ O

27. Oxidation and Reduction of Aldehydes Aldehyde readily undergo oxidation to carboxylic acids. Aldehyde readily undergo oxidation to carboxylic acids. R – C – H ║ O [O] R – C – OH ║ O Aldehyde Carboxylic Acid Among the mild oxidizing agents that convert aldehydes into carboxylic acid is oxygen in air. Thus, aldehydes must be protected from air.

28. Reduction of Aldehydes Aldehydes are easily reduced by hydrogen has (H 2 ) in the presence of a catalysts (Ni, Pt , orm Cu), to form alcohols. The reduction of aldehydes produces primary alcohols. CH 3 – C - H O ║ + H 2 Ni CH 3 – C - H H OH Ethanal Ethanol

29. Primary Alcohol Aldehyde Oxidation Reduction Aldehyde reduction to produce alcohols are opposite of the oxidation of alcohols to produce aldehydes.

30. Reaction of Aldehydes with Alcohols Aldehydes react with alcohols to form hemiacetal and acetals . Reaction with one molecule of alcohol produces a hemiacetal , which is then converted to an acetal by reaction with a second alcohol molecule. Aldehyde + alcohol hemiacetal acid catalyst hemiacetal + alcohol acid catalyst acetal

31. Hemiacetal and acetal formation are very important biochemical reactions, they are crucial to understanding the chemistry of carbohydrates. The Greek prefix hemi- means “half.” When one alcohol molecule has reacted with the aldehyde, the compound is halfway to the final acetal .

32. Hemiacetal Formation Hemiacetal formation is an addition reaction in which a molecule of alcohol adds to the carbonyl group of an aldehyde. The H portion of the alcohol adds to the carbonyl oxygen atom, and R – O portion of the alcohol adds to the carbonyl carbon atom. + R 1 H C ║ O Aldehyde C R 2 H C R 1 R 2 H O H O Hemiacetal

33. Formally defined, a hemiacetal is an organic compound in which carbon atom is bonded to both a hydroxyl group (-OH) and an alkoxy (-OR). The functional group for a hemiacetal is: C OH OR The carbon atom of the hemiacetal functional group is often referred to as the hemiacetal carbon atom: it was the carbonyl carbon atom of the aldehyde that reacted. 4) hemiacetal

34. Indicate whether each of the following compounds is a hemiacetal 1) CH 3 – CH – O – CH 3 OH 2 ) CH 3 – CH – CH – O – CH 3 CH 3 OH O OH 3) 5 ) CH 3 – C – CH 3 O CH 3 OH O CH 3 CH 2 OH 4 )

35. 1. We have an –OH group and an – OR group attached to the same carbon atom. The compound is a hemiacetal . 2) The –OH and –OR groups present in this molecule are attached to different carbon atoms. Therefore, the molecule is not a hemiacetal . 3)We have a ring carbon atom bonded to two oxygen atoms: one oxygen atom in an –OH substituent and the other oxygen atom bonded to the rest of the ring (the same as an R group). This is hemiacetal 4) hemiacetal 5) We have an –OH group and an –OR group attached to the same carbon atom. The compound is a hemiacetal .