Benzene

[Mohit Pandey]

From the name of Benzene(C6H6) , I infer that it is a alkene. But the general formula is CnH2n. The formula does not apply in it. Why?

Try to tell me in simple terms. I have just started reading about Carbon Compunds.

[eric l]

Wikipedia could have told you a lot, e.g. that the 6 C-atoms of benzeneform a closed ring, and that there are alternating single and double bonds - well, at least that was the structure as it was presented by Kekule in 1890.

So, start Googling for "benzene", for "aromatic compounds", for "Kekule", for "styrene" (an other of those compounds that do not fii the the C_nH_2n formula).

[Pyrotex]

From the name of Benzene(C6H6) , I infer that it is a alkene. But the general formula is CnH2n. The formula does not apply in it. Why?...

Benzene is not an alkene. Alkenes are "linear" molecules. Benzene is a "circular" molecule. All carbon compounds containing "rings" are referred to as "Aromatic" hydrocarbons. Benzene is the simplest of the Aromatics.

[Mercedes Benzene]

Ahhhhh Benzene. My favorite arene!

 

But yes. It is not an alkene.

[Mohit Pandey]

Wikipedia could have told you a lot, e.g. that the 6 C-atoms of benzene form a closed ring, and that there are alternating single and double bonds - well, at least that was the structure as it was presented by Kekule in 1890.

So, start Googling for "benzene", for "aromatic compounds", for "Kekule", for "styrene" (an other of those compounds that do not fii the the C_nH_2n formula).

Wikipedia doesn't tell that why Benzene doesn't follow the general formula of alkene.

[Mohit Pandey]

Benzene is not an alkene. Alkenes are "linear" molecules. Benzene is a "circular" molecule. All carbon compounds containing "rings" are referred to as "Aromatic" hydrocarbons. Benzene is the simplest of the Aromatics.

:)

1.Does this linear rule apply to alkane and alkynes also?. Does linear mean straight chain bonds between carbon atoms?

 

2.Benzene is a unsaturated carbon compound . Hence, shouldn't it be alkene?

 

3.The carbon chain with double bonds have suffix "ene" in their name.e.g. ethene. As you say, Benzene is not an alkene. Is there any other compound which this exception?

 

4. Is cychlohexane also aromatic compound?

[eric l]

1. Linear in this context means "non cyclic". Except from the atoms at the head and the tail of the chain, each carbon atom is linked to two others (one on each side). You have also "branched", which means that some carbon atoms are attached to 3 (or 4) other carbon atoms. Such a carbon atom is the "fork" where a new branch stats of.

2. Wiki is incomplete where it states that alkenes have "at least" one double bound, without specifying that with two resp. three double bounds, they would be calle -diene, resp. -triene. Benzene is cyclic (= shows a closed ring) and has 3 double bounds, you might call it "cyclohexatriene"

3. I've never counted them, but there may be more "-enes" that are not alkenes than "-enes" that are. Most aromatic compounds have names in -ene, but there are also products like phosgene, that are not even hydrocarbons ! Do not let the name make you jump to conclusions !

4. Cyclohexane (you have an "h" to much) is NOT an aromatic compound (no double bounds). On the other hand, some aromatic compounds have rings that are not composed of carbon atoms exclusively. e.g. pyridine has a ring with a nitrogen atom in it. And some have double and triple rings. But because of the closed ring, cyclohexane has a formula of the C_nH_2n type.

 

Does that seem complicated ? Well, you are right. But studying chemistry is all about building up structures, molecular structures but also structured insights. Don't try to jump to the fourth level before you have a solid first, second and third level.

[Mohit Pandey]

1. Linear in this context means "non cyclic". Except from the atoms at the head and the tail of the chain, each carbon atom is linked to two others (one on each side). You have also "branched", which means that some carbon atoms are attached to 3 (or 4) other carbon atoms. Such a carbon atom is the "fork" where a new branch stats of.

2. Wiki is incomplete where it states that alkenes have "at least" one double bound, without specifying that with two resp. three double bounds, they would be calle -diene, resp. -triene. Benzene is cyclic (= shows a closed ring) and has 3 double bounds, you might call it "cyclohexatriene"

3. I've never counted them, but there may be more "-enes" that are not alkenes than "-enes" that are. Most aromatic compounds have names in -ene, but there are also products like phosgene, that are not even hydrocarbons ! Do not let the name make you jump to conclusions !

4. Cyclohexane (you have an "h" to much) is NOT an aromatic compound (no double bounds). On the other hand, some aromatic compounds have rings that are not composed of carbon atoms exclusively. e.g. pyridine has a ring with a nitrogen atom in it. And some have double and triple rings. But because of the closed ring, cyclohexane has a formula of the C_nH_2n type.

 

Everthing is cleared . But one more question (I hope it won't cause irritation;))

You said that Benzene is not alkene because it is cyclic in structure.

Alkane like cyclobutane and cyclohexane are also cyclic in nature. If the structure has single bond and is cyclic, can we call it alkane? Further, can we extend this non-cyclic and cyclic rule to alkynes also?

OR if the structure has double bond and is cyclic, it is an aromatic compound. And this rule implies only to the structures of carbon compounds having double bond . Am I correct?

 

Does that seem complicated ? Well, you are right. But studying chemistry is all about building up structures, molecular structures but also structured insights. Don't try to jump to the fourth level before you have a solid first, second and third level.

The structure of Benzene is given in my textbook and on looking its structure, the question came in my mind. :(

[eric l]

No irritation here, just the awareness that there is a lot of explaining to do, and that not all handbooks agree on all points.

 

Let us start with hydrocarbons, general formula c_nH_m.

 

There are two major classes : saturated (= no double or triple bounds), known as alkanes and unsaturated (= at least one double or triple bound). Here, the handbooks start to differ : some classify the alkynes (with a triple bound) among the alkenes (which then means "unsaturated hydrocarbons in general), others see them as entirely different and split the unsaturated hydrocarbons in alkenes and alkynes. Do you follow ? Next, there is the point that there can be more than one double or triple bound, so the formula C_nH_2n is not realy valid for all alkenes.

 

Now, as far as cyclic compounds are concerned, some handbooks classify cyclo-alkanes as a different group, while others classify them within the alkane group. (Same thing for cyclo-alkenes).

 

But aromatic hydrocarbons or arenes are different : their alternating single and double bounds create a kind of electron cloud shared by all the carbon atoms in the ring; talking about "alternating single and double bounds" does not do justice to the real structure. In the case of benzene, the six carbon atoms share 6 electrons (3 pairs from 3 double bounds) in that cloud. So, a ring structure is not sufficient but is absolutely necessary for having an aromatic compound.

Oh, and to complicate matters a little further, an aromatic ring can include other atoms besides carbon. But we better leave that alone for a while.

[Mohit Pandey]

I am writing the things which I understood .

Next, there is the point that there can be more than one double or triple bound, so the formula CnH2n is not realy valid for all alkenes.

Oh! That's why Benzene doesn't follow the general formula of alkenes. and yes , one reason is its alternating single and double bounds.

That also means the general formula of alkenes and alkynes is only for hydrocarbon compounds have single double or triple bonds.Am I correct?

Now, as far as cyclic compounds are concerned, some handbooks classify cyclo-alkanes as a different group, while others classify them within the alkane group. (Same thing for cyclo-alkenes).

But they follow the general formula of alkanes (Cn Hn+2) because they all have one or more single bond.

Benzene is not an alkene. Alkenes are "linear" molecules. Benzene is a "circular" molecule. All carbon compounds containing "rings" are referred to as "Aromatic" hydrocarbons. Benzene is the simplest of the Aromatics.

 

This instigated my doubts for cyclic structure like cyclobutane.

But aromatic hydrocarbons or arenes are different : their alternating single and double bounds create a kind of electron cloud shared by all the carbon atoms in the ring; talking about "alternating single and double bounds" does not do justice to the real structure. In the case of benzene, the six carbon atoms share 6 electrons (3 pairs from 3 double bounds) in that cloud. So, a ring structure is not sufficient but is absolutely necessary for having an aromatic compound.

And this cleared my doubt . Thank you :)

 

Does that seem complicated ? Well, you are right. But studying chemistry is all about building up structures, molecular structures but also structured insights. Don't try to jump to the fourth level before you have a solid first, second and third level.

...........................Oh, and to complicate matters a little further, an aromatic ring can include other atoms besides carbon. But we better leave that alone for a while

 

I have started loving chemistry and all its complications unless it doesn't exceeds my level too much.

[eric l]

Some additional remarks :

1. Single bonds (= 1 pair of electrons shared by two C-atoms) is the standard situation. "One or more single bonds" is a bit off line.
   
2. With double bonds, 2 C-atoms share 2 pairs of electrons; wit triple bonds they share 3 pairs of electrons
   
3. With linear alkanes (saturated hydrocarbons) each C-atom carries 2 H-atoms, except for the C-atoms at the head or tail, where there is room for a third H-atom; hence the formula C_nH_(2n+2)
   
4. In a cyclic saturated hydrocarbon, there is no head or tail, so no C-atom can carry 3 H-atoms; hence the formula is C_nH_2n
   
5. There are also ramified hydrocarbons. At a ramification point, one H-atom is replaced by a side chain, but this side chain has an other tail, so the formula C_nH_(2n+2) remains valid.
   

 

Wikipedia shows structural formulas for different hydrocarbons. Check those if the meaning of the above is not clear.

 

This costs a lot of edditing, with all those subs...

[froster]

Don't know if this was already mentioned or even if it's correct, but I think the word is merely an americanized version of benzin, its original German nomenclature upon discovery, and not really relevant to IUPAC nomenclature of organic compounds.

 

Acetylene is another example of an ene-named non-ene :confused: It's an alkyne.

 

My two cents...