Aldehydes Lecture Presented by: Victor R. OribePresented to:Dr. Leonisa O. Bernardo
Aldehydes contain carbonyl functionalgroup.A carbonyl group is a carbon atom doublebonded to an oxygen atom.The structural representation for acarbonyl group is:
Carbon-oxygen and carbon-carbon double bonddiffer 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 muchgreater than that of carbon (2.5).Hence the carbon-oxygen double bond ispolarized, the oxygen atom acquiring a fractionalnegative charge and the carbon atom acquiring afractional positive charge.
All carbonyl groups have a trigonal planarstructure.The bond angles between the three atomsattached to the carbonyl carbon atom are 1200 ,as would be predicted using VSEPR theory.
An Aldehyde is a carbonyl-containing organiccompound in which the carbonyl carbon atomhas at least one hydrogen atom directly attachedto it.The remaining group attached to the carbonylcarbon atom can be hydrogen, an alkyl group, acycloalkyl group, or an aryl group (Ar).
In interpreting general condensed functionalgroup structure such as RCHO, remember thatcarbon always has four bonds and hydrogenalways has only one. In RCHO, one of carbon’s bonds goes to the R group and one to H; therefore, two bonds must go to O.
Linear notation for an aldehyde functionalgroup and for an aldehyde itself are –CHO andRCHO respectively.Note that the ordering of the symbol H and O inthese notations is HO, not OH (which denotes ahydroxyl group)In an aldehyde, the carbonyl group is alwayslocated at the end of a hydrocarbon chain. O ║ CH3 – CH2 – CH2 – CH2 – C - H
An aldehyde functional group can be bonded toonly one carbon atom because three of the fourbonds from an aldehyde carbonyl carbon mustgo 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 partof a ring system it would have to form two bondsto ring atoms, which would give it five bonds.
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 H manner, removal of a O O hydrogen atom from the ║ –OH group of an alcohol C -2H C and from the carbon atom to which the Aldehyde H hydroxyl group is attached produces aAlcohol carbonyl group.
Physical Properties of AldehydesThe C1 and C2 aldehydes are gases at roomtemperature.The C3 through C11 straight-chain saturatedaldehydes are liquids, and the higher aldehydesare solid.The presence of alkyl groups tends to lower bothboiling points and melting points, as does thepresence of unsaturated in the carbon chain.The boiling point of aldehydes are intermediatebetween those of alcohols and alkanes of similarmolecular mass.
Aldehydes have higher boiling points thanalkanes because of dipole-dipole attractionsbetween molecules.Carbonyl group polarity makes these dipole-dipole interactions possible. Unbranched Aldehydes O C C1 C3 C5 C7 ║ ║ C C2 C4 C6 C8 O gas liquid Dipole-dipole A physical state summary for attraction unbranched aldehydes at room temperature and pressure
Aldehydes have lower boiling points than thecorresponding alcohols because no hydrogenbonding occurs as it does with alcohols.Water molecule can hydrogen-bond withaldehyde molecule. H R H C H O H O O H Aldehyde-water hydrogen bonding
This hydrogen bonding causes low- molecular-mass aldehydes to be water soluble.As the hydrocarbon portion get larger, thewater solubility of aldehydes decreases.Low-molecular –mass aldehyde havepungent, penetrating, unpleasant odors, higher-molecular-mass aldehydes (above C8 ) are morefragrant, especially benzaldehyde derivatives.
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). Cinnamaldehyde cinnamon flavoring Vanillin BenzaldehydeVanilla flavoring almod flavoring
Nomenclature for AldehydeThe IUPAC rules for naming aldehydes:1. Select as the parent carbon chain the longestchain that includes the carbon atom of thecarbonyl group. 1 methane O 2 ethane 5 4 3 2 1║ 3 propane CH3 – CH2 - CH2 – CH – C – H 4 butane Pentane CH2 5 pentane CH2 Parent chain
2. Name the parent chain by changing the –eending of the corresponding alkane to –al. Pentanel a3. Determine the identity and location of anysubstituents, and append this information to thefront of the parent chain name. O 5 4 3 2 1║ 1 methyl CH3 – CH2 - CH2 – CH – C – H 2 ethyl 3 propyl 1 CH2 4 butyl 2 CH3 5 pentyl 2- ethylpentanal
O5 4 3 2 1║CH3 – CH2 – CH – CH2 – C - H OH Hydroxylaldehyde
Assign IUPAC names to the following aldehydes O ║1) CH3 – CH – C- H 2-Methylpropanal CH3 O ║2) CH3 – CH – CH- C - H Cl Cl Dichlorobutanal
O ║3) CH3 – CH2 – CH- C - H 2-Ethylpentanal CH3 – CH2 – CH2 O ║4) H 2-Methylbutanal
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. O O O ║ ║ ║ H–C-H CH3 – C - H CH3 – CH2 – C - HFormaldehyde Acetaldehyde Propionaldehyde
IUPAC system for Naming Aromatic AldehydesAromatic aldehydes are names as derivatives ofbenzaldehyde, the parent compound. O O O ║ ║ ║ C-H Cl C-H C-HBenzaldehyde CH3 3-chloro-5-methylbenzaldehyde OH 4-hydroxybenzaldehyde
Preparation of AldehydesAldehydes can be produced by the oxidation ofprimary alcohol, using mild oxidizing agents such asKMnO4 or K2 Cr2 O7 . OH O ║ oxidation R–C–H R–C–H Aldehyde H Primary When this reaction is used for aldehyde alcohol preparation, reaction conditions must be sufficiently mild to avoid further oxidation of the aldehyde to a carboxylic acid.
The term Aldehyde stems fromalcohol dehydrogenation,indicating that aldehydes arerelated to alcohols by the loss ofhydrogen.
Predicting Products in Alcohol Oxidation ReactionDraw the aldehyde formed from the oxidation of each of thefollowing alcohols. Assume that reaction conditions aresufficiently mild that any aldehydes produced are not oxidizedfurther. O ║ 1) CH3 – CH2 – CH2 - OH CH3 – CH2 – C – H O 2) CH3 – CH – CH2 - OH ║ CH3 – CH – C - H CH3 CH3 CH3 CH3 O 3) CH3 – C – CH2 - OH ║ CH3 – C – C - H CH3 CH3
3) CH3 - CH2 – CH2 – CH2 – CH2 - OH O ║ CH3 - CH2 – CH2 – CH2 – C - H CH3 CH3 O ║4) CH3 - C – CH2 – CH2 – OH CH3 - C – CH2 – C – H CH3 CH3 O5) CH3 – CH2 – CH – CH2 - OH ║ CH3 – CH2 – CH – C – H CH3 CH3
Oxidation and Reduction of AldehydesAldehyde readily undergo oxidation to carboxylicacids.Aldehyde readily undergo oxidation to carboxylicacids. O O ║ [O] ║ R–C–H R – C – OH Aldehyde Carboxylic AcidAmong the mild oxidizing agents that convertaldehydes into carboxylic acid is oxygen in air.Thus, aldehydes must be protected from air.
Reduction of AldehydesAldehydes are easily reduced by hydrogen has (H2 )in the presence of a catalysts (Ni, Pt, orm Cu), toform alcohols.The reduction of aldehydes produces primaryalcohols. O OH ║ Ni CH3 – C - H + H2 CH3 – C - H H Ethanal Ethanol
OxidationPrimaryAlcohol Reduction Aldehyde reduction to produce alcohols areopposite of the oxidation of alcohols to produce aldehydes.
Reaction of Aldehydes with AlcoholsAldehydes react with alcohols to form hemiacetal andacetals.Reaction with one molecule of alcohol produces ahemiacetal, which is then converted to an acetal byreaction with a second alcohol molecule. acidAldehyde + alcohol catalyst hemiacetal acidhemiacetal + alcohol catalyst acetal
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 reactedwith the aldehyde, the compound ishalfway to the final acetal.
Hemiacetal FormationHemiacetal formation is an addition reaction in whicha molecule of alcohol adds to the carbonyl group of analdehyde.The H portion of the alcohol adds to the carbonyloxygen atom, and R – O portion of the alcohol addsto the carbonyl carbon atom. O H O H R1 C O R2 ║ C + C R2 R1 H H Aldehyde Hemiacetal
Formally defined, a hemiacetal is anorganic compound in which carbon atomis bonded to both a hydroxyl group (-OH)and an alkoxy (-OR).The functional group for a hemiacetal is: OH The carbon atom of the C OR hemiacetal functional group is often referred to as the 4) hemiacetal hemiacetal carbon atom: it was the carbonyl carbon atom of the aldehyde that reacted.
Indicate whether each of the following compounds is a hemiacetal OH1) CH3 – CH – O – CH3 CH2 4) O CH3 OH CH32) CH3 – CH – CH – O – CH3 OH OH 5) CH3 – C – CH3 O OH3) O CH3
1. We have an –OH group and an – OR group attached to thesame carbon atom. The compound is a hemiacetal.2) The –OH and –OR groups present in this molecule areattached to different carbon atoms. Therefore, the moleculeis not a hemiacetal.3)We have a ring carbon atom bonded to two oxygen atoms:one oxygen atom in an –OH substituent and the otheroxygen atom bonded to the rest of the ring (the same as anR group). This is hemiacetal4) hemiacetal5) We have an –OH group and an –OR group attached to thesame carbon atom. The compound is a hemiacetal.