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My book says geminal dihalide or alkylidene halide is the common name for dihaloalkanes where both halogen atoms(if of same element) are bonded to the same carbon atom and alkylene dihalide if 2 halogens of same element bonded to adjacent Carbon atoms.

Common name

But in a list of examples in the same book, $\ce{CH2Cl2}$ is named as Methylene Chloride. Shouldn't it be Methylidene Chloride?

Dichloromethane

Here is text from NCERT Chemistry Class 12 Part 2 Chapter 10:

The dihaloalkanes having the same type of halogen atoms are named as alkylidene or alkylene dihalides. The dihalo-compounds having both the halogen atoms are further classified as geminal halides or gem-dihalides when both the halogen atoms are present on the same carbon atom of the chain and vicinal halides or vic-dihalides when halogen atoms are present on adjacent carbon atoms. In common name system, gem-dihalides are named as alkylidene halides and vic-dihalides are named as alkylene dihalides. In IUPAC system, they are named as dihaloalkanes.

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    $\begingroup$ If your book calls C2H2 methylene chloride you need to get a better book $\endgroup$
    – Waylander
    Jun 28, 2023 at 13:52
  • $\begingroup$ @Waylander sorry I made a typo! $\endgroup$ Jun 28, 2023 at 16:10
  • $\begingroup$ Even after correction, the formula of methylene chloride is not $\ce{C2H2Cl2}$. Methylene chloride is $\ce{CH2Cl2}$, with $1$ carbon atom. $\endgroup$
    – Maurice
    Jun 28, 2023 at 16:17
  • $\begingroup$ @Maurice sorry again, I made a typo in the edit too. The title was correct only $\endgroup$ Jun 28, 2023 at 16:26

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No ! $\ce{C2H2}$ (from $4$th line) is named ethyne. It has also a more usual and older name : acetylene. But today the author wants to discuss about $\ce{CH2Cl2}$

$\ce{CH2Cl2}$ is usually called dichloromethane. But it has an older name : methylene chloride, which can be justified by stating that $\ce{CH2Cl2}$ looks as it was a chloride (and even a dichloride) of the group $\ce{CH2}$ who was earlier called methylene. This nomenclature was coming from before IUPAC (created $1918$). Before $1918$, all compounds of chlorine were called "chlorides".

In the $19$th century, the alkanes had an end in -ane (like today). With one $\ce{H}$ removed, the molecule becomes a radical in -yl (like today). But with two $\ce{H}$ lost, it becomes -ene or -ylene. Today, we specify on which atom the $\ce{H}$ atoms have been removed, in molecules containing $2$ or more Carbon atoms. Strangely enough, removing $3$ $\ce{H}$ atoms from an alkane gave the group $\ce{CH}$ which was sometimes called form- (from formic acid).

Let's go back to all chloro- derivates of methane: Removing $1$ or several $\ce{H}$ atoms from methane $\ce{CH4}$ yields first the radical $\ce{CH3}$ which was and is called methyl; then $\ce{CH2}$ is obtained which was called methylene. So, before IUPAC, $\ce{CH3Cl}$ was called methyl chloride (today chloromethane), and $\ce{CH2Cl2}$ was called methylene chloride (today dichloromethane). Furthermore, trichloromethane $\ce{CHCl3}$ was called "chloroform", and tetrachloromethane $\ce{CCl4}$ was called "carbon tetrachloride".

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    $\begingroup$ Methylene chloride is still widely used among lab chemists so students of chemistry should be aware of this name $\endgroup$
    – Waylander
    Jun 28, 2023 at 13:54
  • $\begingroup$ Nice history lesson in there. Chemistry retains many of these older names. $\endgroup$
    – Jon Custer
    Jun 28, 2023 at 14:45
  • $\begingroup$ So, is this an exception to the "Alkylidene halide & Alkylene dihalide" thing ? $\endgroup$ Jun 28, 2023 at 16:16
  • $\begingroup$ What's the meaning of : *Is this an exception to ... ?" Which "this" is it ? $\endgroup$
    – Maurice
    Jun 28, 2023 at 16:21
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    $\begingroup$ Maybe your book is right. I don't know. But what I know is the fact that $\ce{CH2Cl2}$ is called and sold under the name methylene chloride or dichloromethane. Never methylidene chloride. Anyway this ending in -idene is never used. It is better to specify the number of the C atom in the alkane chain where the chlorine atoms are fixed. $\endgroup$
    – Maurice
    Jul 1, 2023 at 15:43

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