How will you synthesize DDT from chlorobenzene



The name D.ichlorddiphenyltRichloroethane (DDT) is a bit lax for a number of reasons. Correct is


DDT is made from chloral and chlorobenzene. The mechanism of synthesis is a typical electrophilic aromatic substitution in which the chloral is the electrophilic agent. Its carbonyl group is so reactive because of the electron-withdrawing chlorine atoms that it is normally present as chloral hydrate and previously with conc. Sulfuric acid needs to be dehydrated into the active form. The reaction itself also takes place in a strongly acidic medium.

Activation of the carbonyl component

Electrophilic attack

The chlorine deactivates the benzene ring against electrophilic attack (strong -I effect, hence the relatively drastic reaction conditions here), but directs second substituents into the because of the possibility of mesomerism (+ M effect) para, or. ortho-Position:

This is due to the fact that only when attacking in the para and also in the ortho-Position for the σ-complex a fourth limit formula can be written, in which the charge is on the chlorine atom. (See the limit formula marked in color.) The better distribution of the positive charge is the reason why this state is more energy-efficient and therefore kinetically preferred. The σ-complex reacts with elimination of H+ to a first intermediate product:

Due to the acidic conditions, the hydroxyl function is protonated and then eliminated as water. This is the typical activation of alcohols in nucleophilic substitution reactions. (see e.g. representation of symmetrical ethers by acid-catalyzed condensation of alcohols) The electrophile thus obtained reacts with a second molecule of chlorobenzene according to the same mechanism:

Since there is only one for each aromatic nucleus paraPosition and two orthoPositions, one should statistically expect a mixture, which is predominantly from orthoProducts. However, steric reasons urge that ortho- Share back to a maximum of 25%. (Tip: Draw the structure in a molecule editor and display a 3D dome representation to get a feel for the space requirements of the atoms.)


DDT is an extremely effective insecticide. It is not very toxic to mammals. After the Second World War, various epidemics were combated or even eradicated, including malaria, typhus, yellow fever (Anopheles mosquito), sleeping sickness (tse-tse fly) as well as typhus, plague, cholera and the Colorado beetle. DDT saved millions of lives in the process. Paul Hermann Müller, the discoverer of the insecticidal effect, received the Nobel Prize for this in 1948. Unfortunately, DDT also has serious disadvantages:

  • It has a non-specific effect, i.e. it is also toxic to bees, for example.
  • It persists in the environment for too long. (Half-life in subtropical areas: 0.06 to 1 year, temperate climates: 2.3 to 16.7 years)
  • It is non-polar and accumulates in adipose tissue and thus also in the food chain. (Example: From marine plankton to fish-eating birds: up to a factor of 85,000)
  • Insects can also develop resistance to DDT.

It was soon discovered that DDT also has hormone-like effects. This is particularly likely due to the admixtures of ortho-Isomers, whose hormonal effect is considered to be significantly greater. The suspected carcinogenic potential is probably also based on the hormonal effects of the ortho-Isomers. A reduction in the thickness of the eggshell of birds of prey and the associated reduction in the population as well as the detection of DDT in various species and the detection of DDT in breast milk were further alarm signals to ban the use of DDT in the 1970s, even if in humans until then no effects were discernible.

Since the ban on DDT, however, the spread of malaria has increased sharply again because all other insecticides have proven to be insufficiently effective. The WHO has therefore restricted the use of DDT again.

Questions in chemistry class

DDT is therefore well suited for ethical questions in chemistry:

  • Should you allow DDT? If so, where and for which application or in what maximum amount?
  • Should there be more research into how the hazard potential is distributed among the various isomers? If so: who should do it and who should finance it?
  • If it turns out that the para- / para-Isomer is significantly less toxic: How could one - ideally worldwide - force the - hitherto technical - synthesis to be improved in such a way that less? ortho-Isomers are obtained as a by-product? How could the higher price due to the higher synthesis effort be enforced on the market?

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