Benzaldehyde

• Benzencarbaldehyd (IUPAC)
• Benzolcarbaldehyd
• Phenylmethanal
• Artificial oil of bitter almonds

Colorless to yellowish liquid with odor bittermandelartigem

Liquid

1.05 g · cm -3 ( 20 ° C)

-26 ° C

179 ° C

1.26 hPa (20 ° C)

3 g · l-1 (20 ° C ) in water

1.5446 (20 ° C)

Attention

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Benzaldehyde [ bɛnt͡s | aldehydes ː t] (also called " artificial oil of bitter almonds " ) is a colorless to yellowish liquid with bittermandelartigem odor. Benzaldehyde is the simplest aromatic aldehyde. It is derived from benzene and is chemically and structurally closely related to benzoic acid, which can be obtained from it by oxidation. He is an important basic chemical.

Representation

• Free-radical chlorination of toluene gives benzal chloride ( Dichlormethylbenzol ) provided with water to benzaldehyde ( and HCl) can be implemented. Corresponding benzaldehyde can also be obtained by free-radical bromination of toluene, followed by hydrolysis. In both cases occurs in the last reaction step is a nucleophilic substitution of halide.
• A ' direct ' oxidation of toluene to benzaldehyde is possible. Since benzaldehyde but is easily oxidized to benzoic acid, but either selective oxidizing agents have to be used or shall be ' caught ' with a quick response and thus protected from further oxidation of the resulting benzaldehyde.

Properties

Benzaldehyde is injurious to health, with a real health hazard for usual can be expected only in comparatively large quantity ingested. It smells pleasantly sweet marzipan. The taste of the benzaldehyde is generally, but felt to be characteristic marzipan -like in its pure state as unpleasant burning. In very dilute solutions, in this case especially with ethanol, occurs increasingly a wild cherry notes added to the flavor. Benzaldehyde is also one of the flavors in the wine. With an odor threshold value of 3 mg / l in white wine, it is an important component of the wine bouquets. Benzaldehyde has a flash point of 64 ° C and an ignition temperature of 190 ° C.

Reactions

• Benzaldehyde C6H5 -CHO is easily oxidized to benzoic acid C6H5 -COOH. This reaction takes place - albeit very slowly - even at room temperature and atmospheric oxygen ( autoxidation ), so that benzaldehyde is often contaminated with benzoic acid (for larger quantities as a white solid in liquid aldehyde). As an intermediate step here peroxy benzoic acid is formed.
• Benzaldehyde is for aldehydes typical reactions, which are also suitable for detection. It reacts with hydrazine, for example H 2 N- NH 2 and its derivatives ( for example, with phenylhydrazine C6H5 -NH- NH2 to form a phenylhydrazone ).
• Typical of the aromatic electrophilic substitution is also possible with the benzaldehyde. In the nitration, the reaction conditions must be carefully chosen because the oxidation to benzoic acid occurs as a side reaction. For this reason the yield of the nitrobenzaldehyde is usually less than 50%.
• Benzoin Addition: Two molecules of benzaldehyde can unite in the presence of cyanide as a catalyst to benzoin. Generally understood as the benzoin addition the corresponding reaction of aromatic aldehydes ( benzaldehyde as the simplest representative).
• Aldol Condensation: This refers generally to the addition of carbonyls to aldehydes. As a simple example dibenzalacetone represent. Here, a ketone ( acetone) reacts once common in basic to a so-called methylene component. This reacts after addition of benzaldehyde ( carbonyl ) to benzalacetone ( benzylidene ). Performs one another by adding benzaldehyde by, the benzalacetone respond this time to the methylene component in a basic medium. By the second addition or aldol condensation double complex molecules can be synthesized, such as the dibenzalacetone (1,5- Diphenylpenta -1 ,4 -dien- 3-one).
• In the hydrate - forming an addition of water to the aldehyde group of the carbonyl compound under nucleophilic attack takes place. A disadvantage of this reaction is the instability of generally hydrates.
• In the reaction with primary amines (R- NH2) also form under nucleophilic attack at the central carbon atom of the carbonyl group and subsequent elimination of water (condensation) from the intermediate azomethine.