Organic Syntheses, CV 3, 127
α-BROMOHEPTALDEHYDE
Submitted by Paul Z. Bedoukian
Checked by Joseph A. Pappalardo and Charles C. Price.
1. Procedure
Caution! Since most of the reactants are of unpleasant odor or have lachrymatory effects it is best to use a good hood.
A.
Heptaldehyde enol acetate. A mixture of
285 g. (335 ml., 2.5 moles) of heptaldehyde (Note
1),
612 g. (566 ml., 6 moles) of acetic anhydride, and
49 g. (0.5 mole) of powdered potassium acetate is placed in a
2-l. flask fitted with a reflux condenser. The flask is heated in an
oil bath kept at 155–160° for 1 hour. The mixture is then allowed to cool, placed in a
2-l. separatory funnel, washed several times with warm water (Note
2) to remove the excess
acetic anhydride, and finally washed with
5% sodium carbonate solution (Note
3). The resultant oil is fractionated under reduced pressure through an
efficient column (Note
4). The initial fraction consists of pure
heptaldehyde followed by
heptaldehyde containing
heptaldehyde enol acetate. The fraction boiling at
88–90°/17 mm. is pure enol acetate,
nD25 1.4295–1.4305;
d425 0.880–0.884. The yield is
175–195 g. (
45–50%) (Note
5).
B.
α-Bromoheptaldehyde dimethyl acetal. A solution of
156 g. (177 ml., 1 mole) of the enol acetate and
200 ml. of carbon tetrachloride is placed in a
1-l. flask and cooled in an
ice-water bath. A mixture of
160 g. (51 ml., 1 mole) of bromine and
50 ml. of carbon tetrachloride is added slowly through a
buret, the flask being constantly shaken and the rate of addition so controlled as not to allow the temperature of the brominated mixture to rise above 10° (Note
6). The addition of
bromine takes from 20 minutes to 1 hour, and the end point is reached when the calculated amount is absorbed and the
bromine is no longer decolorized. The brominated mixture is added to
600 ml. of anhydrous methanol (Note
7) and allowed to stand for 48 hours or longer. At the end of this period the mixture is diluted with 2 l. of water and the separated oil (lower layer) is washed with 1 l. of water and finally with
1 l. of 5% sodium carbonate (Note
8). The
carbon tetrachloride and
methyl acetate are removed by distillation at atmospheric pressure. The residual oil is then distilled under reduced pressure in the presence of a small amount of
sodium carbonate. The fraction boiling at
117–119°/17 mm. is collected as pure
α-bromoheptaldehyde dimethyl acetal,
nD25 1.4510–1.4520;
d425 1.180–1.195. The yield is
191–203 g. (
80–85%) (Note
9).
C.
α-Bromoheptaldehyde. A mixture of
119.5 g. (100 ml., 0.5 mole) of α-bromoheptaldehyde dimethyl acetal and
80 ml. of concentrated hydrochloric acid is boiled gently in a
250-ml. distilling flask, and the
methanol liberated is removed by distillation, which is continued slowly until the vapor temperature reaches 90°, at which point the heating is stopped and the residue and distillate are combined and diluted with 200 ml. of water. The somewhat brownish oil which separates is distilled under reduced pressure from a
250-ml. Claisen flask. The yield of pure
α-bromoheptaldehyde, boiling at
87–92°/17 mm.,
nD25 1.4580–1.4600,
d425 1.210–1.230, is
87–92.5 g. (
90–95%).
2. Notes
1. The
heptaldehyde should be a freshly distilled product boiling at
151.5–153.5°.
2. Decomposition of the excess
acetic anhydride takes place very slowly and with difficulty unless warm water is used. The checkers used three portions of wash water at 40–50°, totaling 1.3 l.
4. The reaction mixture consists of unchanged
heptaldehyde,
heptaldehyde enol acetate,
heptaldehyde diacetate, and a small amount of polymerized material. The proportion of free
heptaldehyde and
heptaldehyde diacetate depends upon the time of heating, longer periods of heating favoring the formation of the diacetate. An
efficient fractionating column, preferably of the Whitmore-Fenske type, should be used in order to obtain the enol acetate free of the
heptaldehyde and
heptaldehyde diacetate impurities. The checkers used a
Whitmore-Fenske type column of about six theoretical plates.
5. The residue consists largely of
heptaldehyde diacetate, which when slowly distilled at atmospheric pressure partially decomposes to
acetic anhydride and
heptaldehyde. In this manner, 50–60% of the available
heptaldehyde is recovered from the residue.
6. The checkers recommend a
mechanical stirrer to avoid danger of contact with
bromine.
7. Commercial
methanol (99.5–100%) was used in all experiments.
8. The acetal must be free of acid; otherwise decomposition takes place during distillation.
9. The pure bromoacetal is a stable, colorless liquid, of mild odor. It may be kept indefinitely when stored in a dark bottle over a small amount of
anhydrous sodium carbonate.
3. Discussion
The procedure described is an example of a general method of preparation of α-bromoaldehydes.
1 α-bromoheptaldehyde has been prepared by the bromination of
heptaldehyde diethyl acetal with
phosphorus trichlorodibromide2 and by direct bromination of heptaldehyde trimer with subsequent treatment with alcohol.
3
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