Physical properties
At room temperature, in addition to formaldehyde as a gas, aliphatic aldehydes with less than 12 carbon atoms in the molecule are liquids, higher aldehydes are solids, and aromatic aldehydes are liquids or solids. Low-grade fatty aldehydes have a strong pungent odor. Aldehydes with 9 carbon atoms in the molecule and 10 carbon atoms in the molecule have a floral and fruity fragrance, so they are often used in the perfume industry.
Due to the polarity of the carbonyl group, the boiling point of aldehydes is higher than that of hydrocarbons and ethers with similar molecular weights. However, since hydrogen bonds cannot be formed between carbonyl molecules, the boiling point is lower than that of the corresponding alcohols.
Because the carbonyl group of aldehydes can form hydrogen bonds with hydrogen in water, lower aldehydes are soluble in water; however, aromatic aldehydes are generally difficult to dissolve in water.

Chemical nature
In organic reactions, the reaction of hydrogenation or deoxygenation is called reduction reaction. Acetaldehyde catalyzes hydrogenation to produce ethanol, which occurs in the carbonyl group. The π bond in C=O is broken to form a CO single bond (in the carbon-oxygen double bond, one is π bond, one is σ bond, π bond is more active and easy to break; σ bond is relatively stable), acetaldehyde is reduced; the reaction of dehydrogenation or oxygen addition is called oxidation reaction, and acetaldehyde is easily oxidized to acetic acid. The aldehyde group CH is broken to form C-OH, and acetaldehyde is oxidized.
Aldehydes usually have strong reducibility and a certain degree of oxidation.
The silver mirror reaction of formaldehyde is: HCHO + 4Ag(NH3)2OH CO2↑+ 8NH3+ 4Ag↓+3H2O [phenomenon: bright silver mirror appears on the inner wall of the test tube]
R-CHO + 2Ag(NH3)2OH R-COONH4 + 2Ag↓ + 3NH3 + H2O
Reaction with freshly prepared copper hydroxide (Fehling's reagent, Ban's reagent, Benedict's reagent): [phenomenon: brick red precipitate appears]
R-CHO + 2Cu(OH)2 R-COOH + Cu2O↓ + 2H2O
Reaction with bromine water: R-CHO + Br2 + H2O R-COOH + 2HBr
Addition reaction: R-CHO + H2 R-CH2-OH
2R-CHO+O22R-COOH
Formaldehyde and phenol undergo polycondensation reaction to produce phenolic resin.
Reaction law
The aldehyde group is polar, and the oxygen atom is the negative electrode in the carbon-oxygen bond, pulling the electron of the carbon atom to the oxygen atom. Due to the structural characteristics of aldehydes, the π bond in the carbonyl group is polarized, so that the oxygen atoms are partially negatively charged, and the carbon atoms are partially positively charged. In the reaction, the carbon-oxygen double bond in the molecule is easily attacked by the negatively charged reagent, that is, the nucleophile, and the reaction occurs.
In addition, under the influence of the carbonyl group, the hydrogen atom on the carbon atom directly connected to the carbonyl group is very active and can undergo a series of reactions. Therefore, the nucleophilic addition of carbonyl groups and the reactivity of adjacent hydrogen atoms are the main reactions of aldehydes.

Classification
According to hydrocarbon group
Aldehydes can be divided into fatty aldehydes, ester cyclic aldehydes, aromatic aldehydes and terpene aldehydes. Fatty aldehyde refers to an aldehyde in which the carbon atoms in the molecule are connected to form a chain, which is open-chain. Alicyclic aldehyde refers to the carbon atoms in the molecule connected to form a closed carbocyclic ring. The carbonyl group of the aromatic aldehyde is directly attached to the aromatic ring. Terpene aldehydes are a branch of terpenoids.
1. Aliphatic compound refers to the carbon chain formed by the mutual bonding of carbon atoms in the molecule, which is not ring-shaped. Fatty aldehydes are a classification of fatty compounds.
Common acyclic fatty aldehydes are: octyl aldehyde, nonanal, decanal, undecyl aldehyde, lauric aldehyde (dodecanal), tridecyl aldehyde, myristic aldehyde (tetradecanal), methylhexyl acetaldehyde, methyl Octylacetaldehyde, methylnonylacetaldehyde, trimethylhexanal, tetramethylhexanal, trans-2-hexenal, 2-nonenal, trans-4-decenal, undecenal , Nonadienal and so on.
2. The cycloaliphatic compound can be regarded as obtained by connecting and closing the open-chain compound to form a ring. Alicyclic aldehydes are a classification of alicyclic compounds.
Common alicyclic aldehydes are: lignan aldehyde, ivy aldehyde, isocyclic citral, citrin aldehyde, methyl citrin aldehyde, new lily aldehyde and so on.
3. The carbonyl group of aromatic aldehyde is directly connected to the aromatic ring. This type of aldehyde can be regarded as a derivative of benzene.
Common aromatic aldehydes are: benzaldehyde, phenylacetaldehyde, phenylpropanal, cinnamic aldehyde, lily of the valley, vanillin, ethyl vanillin and so on.
4. Terpene aldehyde refers to a classification of terpenoids. Terpenoids are derivatives with the general formula (C5H8)n and their oxygen content and different degrees of saturation.
Common terpene aldehydes are: citral, citronellal, hydroxycitronellal, perillaldehyde, trimethylheptenal and so on.
By functional group
Aldehydes can be divided into mono-, di- and poly-aldehydes.
According to the degree of saturation
Aldehydes can be divided into saturated aldehydes and unsaturated aldehydes.

Application and discovery
Important aldehydes and related compounds. From left to right: formaldehyde and trioxane, acetaldehyde and its enol form, glucose (pyranose), food flavor cinnamaldehyde and vitamin B6.
Many traces of aldehydes are found in essential oils due to their aromatic odor, such as: cinnamaldehyde, coriander aldehyde and vanillin. Perhaps due to the high activity of formyl, aldehyde groups are rare in natural products (amino acids, nucleic acids, oils). Most sugars are aldehyde derivatives. These "aldoses" generally exist in the form of hemiacetals, and a few exist in the form of aldehydes. For example, a small part of glucose in aqueous solution is in the form of aldehydes.

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