All that glitters is not butane. Linear alkenes and alkynes

In fact, if it glitters it is surely not butane. Anyway, I meant there’s more than alkanes in organic chemistry. For example, do you know what butene and butyne are?

If an alkane is a saturated hydrocarbon, an alkene is an unsaturated hydrocarbon (some carbon atoms are bonded to fewer than four other atoms). The unsaturation of the alkenes is, more exactly, a double bond, a bond between two carbon atoms that counts as two bonds, and that’s the reason why they can’t be bonded to four other atoms. We’re going to have a look at the simplest akenes, as last time, the linear ones.

1-butè (font)

1-butene (source)

Butene, with four carbon atoms, can be represented with the formula CH2=CH−CH2−CH3 or C4H8. Notice that both carbon atoms taking part in the double bond are connected to just three other atoms. However, butene is not only this. The formula CH3−CH=CH−CH3 also represents a butene, it can also be reduced to C4H8.

These two compounds are 1-butene and 2-butene, respectively, two isomers of butene, that is, two compounds represented by the same formula, but which are different somehow. Both of them are, nevertheless, butenes since they are alkenes with a linear chain of four carbon atoms. Numbers 1 and 2 tell in which carbon atom the double bond begins. 3-Butene doesn’t exist for it would be 1-butene back-to-front and the lower number is preferred.

1-butí (font)

1-butyne (source)

In an alkyne, however, the unsaturation of the hydrocarbon is a triple bond, which counts as three bonds. Now we have 1-butyne and 2-butyne, CH≡C−CH2−CH3 and CH3−C≡C−CH3, respectively. Their general formula is C4H6 and the carbon atoms with the triple bond are bonded to two other atoms each.

The general formula for alkenes is CnH2n and the one for alkynes is CnH2n2. The members of each family of compounds is named with the prefix of the corresponding number of carbon atoms in their chains and the -ene or -yne ending according to the family they belong to.

Nomenclature and formulas of some alkenes and alkynes.

English Catalan Spanish Portuguese extended formula or isomers1 general formula
alkene alquè alqueno alceno2 CnH2n
ethene etè eteno eteno CH2=CH2 C2H4
propene propè propeno propeno CH2=CH−CH3 C3H6
butene butè buteno buteno 2 isomers C4H8
pentene pentè penteno penteno 3 isomers C5H12
octene octè octeno octeno 6 isomers C8H16
alkyne alquí alquino alcino2 CnH2n − 2
ethyne etí etino etino CH≡CH C2H2
propyne propí propino propino CH≡C−CH3 C3H4
butyne butí butino butino 2 isomers C4H6
pentyne pentí pentino pentino 3 isomers C5H8
octyne octí octino octino 6 isomers C8H10

1 Geometry not taken into account.
2Porquê ‘alceno’, ‘alcino’ e ‘alquilo’ em lugar de ‘alqueno’, ‘alquino’ e ‘alcilo’?”, Bernardo Jerosch Herold


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