3 added 114 characters in body
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There is only one possible phenylacetaldehyde, so its IUPAC name is in fact phenylacetaldehyde. 2-Phenylacetaldehyde is its systematic IUPAC name, but many chemicals' official and systematic IUPAC names differ, like propan-2-one for example. The reasoning for this is often historical and to reflect what is most commonly used in the scientific community.

$\hspace{4.9cm}$chloro molecules

Chloroacetaldehyde is quite obviously an aldehyde, but switching the position of the hydrogen and the chlorine as you suggested results in a completely different class of molecule known as an acyl chloride. There is therefore only one possible chloroacetaldehyde and no need to specify the chlorine's location, though its systematic IUPAC name is indeed 2-chloroethanal.

$\hspace{6.2cm}$enter image description here

There is only one possible phenylacetaldehyde, so its IUPAC name is in fact phenylacetaldehyde. 2-Phenylacetaldehyde is its systematic IUPAC name, but many chemicals' official and systematic IUPAC names differ, like propan-2-one for example.

$\hspace{4.9cm}$chloro molecules

Chloroacetaldehyde is quite obviously an aldehyde, but switching the position of the hydrogen and the chlorine as you suggested results in a completely different class of molecule known as an acyl chloride. There is therefore only one possible chloroacetaldehyde and no need to specify the chlorine's location, though its systematic IUPAC name is indeed 2-chloroethanal.

$\hspace{6.2cm}$enter image description here

There is only one possible phenylacetaldehyde, so its IUPAC name is in fact phenylacetaldehyde. 2-Phenylacetaldehyde is its systematic IUPAC name, but many chemicals' official and systematic IUPAC names differ, like propan-2-one for example. The reasoning for this is often historical and to reflect what is most commonly used in the scientific community.

$\hspace{4.9cm}$chloro molecules

Chloroacetaldehyde is quite obviously an aldehyde, but switching the position of the hydrogen and the chlorine as you suggested results in a completely different class of molecule known as an acyl chloride. There is therefore only one possible chloroacetaldehyde and no need to specify the chlorine's location, though its systematic IUPAC name is indeed 2-chloroethanal.

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2 corrected flagrant errors
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$\hspace{4.7cm}$phenylacetaldehydes$\hspace{6.2cm}$enter image description here

Both 1-phenylacetaldehyde (acetophenone) and 2-phenylacetaldehyde are aldehydes.There is only one possible phenylacetaldehyde, so its IUPAC convention specifies the 2 to reduce uncertaintyname is in which aldehyde you are referring tofact phenylacetaldehyde.

In actuality, however, since 1-phenylacetaldehyde is most commonly known as acetophenone, you could get away with calling 2-phenylacetaldehyde just plain ol' phenylacetaldehyde. That's whatPhenylacetaldehyde is its wikipedia page'ssystematic IUPAC name is after all, but many chemicals' official and systematic IUPAC names differ, like propan-2-one for example.

$\hspace{4.9cm}$chloro molecules

Chloroacetaldehyde is quite obviously an aldehyde, but switching the position of the hydrogen and the chlorine as you suggested results in a completely different class of moleculesmolecule known as an acyl chlorideschloride. There is therefore only one possible chloroacetaldehyde and no need to specify the chlorine's location, though its systematic IUPAC name is indeed 2-chloroethanal.

$\hspace{4.7cm}$phenylacetaldehydes

Both 1-phenylacetaldehyde (acetophenone) and 2-phenylacetaldehyde are aldehydes. IUPAC convention specifies the 2 to reduce uncertainty in which aldehyde you are referring to.

In actuality, however, since 1-phenylacetaldehyde is most commonly known as acetophenone, you could get away with calling 2-phenylacetaldehyde just plain ol' phenylacetaldehyde. That's what its wikipedia page's name is after all.

$\hspace{4.9cm}$chloro molecules

Chloroacetaldehyde is quite obviously an aldehyde, but switching the position of the hydrogen and the chlorine as you suggested results in a completely different class of molecules known as acyl chlorides. There is therefore only one possible chloroacetaldehyde and no need to specify the chlorine's location.

$\hspace{6.2cm}$enter image description here

There is only one possible phenylacetaldehyde, so its IUPAC name is in fact phenylacetaldehyde. 2-Phenylacetaldehyde is its systematic IUPAC name, but many chemicals' official and systematic IUPAC names differ, like propan-2-one for example.

$\hspace{4.9cm}$chloro molecules

Chloroacetaldehyde is quite obviously an aldehyde, but switching the position of the hydrogen and the chlorine as you suggested results in a completely different class of molecule known as an acyl chloride. There is therefore only one possible chloroacetaldehyde and no need to specify the chlorine's location, though its systematic IUPAC name is indeed 2-chloroethanal.

    Post Deleted by ringo
1
source | link

$\hspace{4.7cm}$phenylacetaldehydes

Both 1-phenylacetaldehyde (acetophenone) and 2-phenylacetaldehyde are aldehydes. IUPAC convention specifies the 2 to reduce uncertainty in which aldehyde you are referring to.

In actuality, however, since 1-phenylacetaldehyde is most commonly known as acetophenone, you could get away with calling 2-phenylacetaldehyde just plain ol' phenylacetaldehyde. That's what its wikipedia page's name is after all.

$\hspace{4.9cm}$chloro molecules

Chloroacetaldehyde is quite obviously an aldehyde, but switching the position of the hydrogen and the chlorine as you suggested results in a completely different class of molecules known as acyl chlorides. There is therefore only one possible chloroacetaldehyde and no need to specify the chlorine's location.