The first lessons of organic chemistry, in general, confuse the student who often believes – wrongly – that the numerous formulas from which he is bombarded are to be learned by heart and not to be obtained by applying simple rules. For many years I have overcome the natural rejection of students by exposing, before starting the systematic study of the various functions, how formulas are obtained and names are assigned to the following classes of compounds: hydrocarbons, alcohols, aldehydes and ketones, carboxylic acids.
To overcome the difficulties of students in the face of cases of isomerism, it would be appropriate to limit oneself, at first, to the illustration of the molecular formulas, underlining how only the first hydrocarbons of the various series retain ancient names to be memorized, while for all the others they adopt the usual prefixes penta- , hexa- , hepta- , etc. and the suffixes -ane for alkanes and cycloalkanes , -ene for alkenes , cycloalkenes and arenes , -ino for alkynes .
Alkanes ( C n H 2n + 2 ): CH 4 , methane; C 2 H 6 , ethane; C 3 H 8 , propane; C 4 H 10 , butanes.
Alkenes ( C n H 2n ): C 2 H 4 , ethene; C 3 H 6 , propene; C 4 H 8 , butenes.
Alkynes ( C n H 2n-2 ): C 2 H 2 , ethin; C 3 H 4 , propine; C 4 H 6 , butynes.
In a second time, after having premised that the methyl grouping CH 3 formally results from methane CH 4 by subtracting a hydrogen atom, we could explain the rule: to write the rational formula of a hydrocarbon one replaces a hydrogen atom with a methyl in the formula of that which precedes it in the series. Students will quickly notice how substitution is often possible at different points in the molecule and will discover for themselves the existence of isomers.
The methane CH 4 is followed by a single ethane CH 3 CH 3 and to these a single propane CH 3 CH 2 CH 3 .
Al propane followed by two butanes : the butane CH 3 (CH 2 ) 2 CH 3 and 2-methylpropane CH 3 CH (CH 3 ) CH 3 ( iso butane).
The butane is followed by two pentanes : pentane CH 3 (CH 2 ) 3 CH 3 and 2-methylbutane CH 3 CH (CH 3 ) CH 2 CH 3 ( iso pentane). The isobutane is followed by a third pentane, 2,2-dimethylpropane C (CH 3 ) 4 ( neo pentane).
All ‘ ethylene CH 2 = CH 2 follows a single propene CH 2 = CHCH 3 , and this three butenes: 1-butene, CH 2 = CHCH 2 CH 3 , 2-methylpropene CH 2 = CHC (CH 3 ) 2 and 2-butene CH 3 CH = CHCH 3 .
They can be defined as products of the formal substitution, in a hydrocarbon, of hydrogen atoms with hydroxyls -OH . Their names are those of hydrocarbons with the suffixes -olo, -diol , -triolo , etc.
From methane CH 4 (or HCH 3 ) derives only one alcohol, methanol CH 3 OH (or HCH 2 OH).
From ethane CH 3 CH 3 derive ethanol CH 3 CH 2 OH and ethanediol HO (CH 2 ) 2 OH (ethylene glycol).
Two monols derive from propane CH 3 CH 2 CH 3 , 1-propanol CH 3 CH 2 CH 2 OH and 2-propanol CH 3 CH (OH) CH 3 ( iso propanol); two diols , 1,2-propanediol CH 3 CH (OH) CH 2 OH and 1,3-propanediol HO (CH 2 ) 3 OH (trimethylene glycol); a triol, propanetriol HOCH 2 CH (OH) CH 2 OH (glycerol).
Four monols derive from butane CH 3 (CH 2 ) 2 CH 3 and isobutane CH 3 CH (CH 3 ) CH 3 , 1-butanol CH 3 (CH 2 ) 2 CH 2 OH , 2-butanol CH 3 CH (OH) CH 2 CH 3 ( secbutanol ), 2-methyl-1-propanol CH 3 CH (CH 3 ) CH 2 OH ( isobutanol ) and 2-methyl-2-propanol CH 3 C (CH 3 ) (OH) CH 3 terbutanol).
Also from butane derive three butanediols , 1,2-butanediol CH 3 CH 2 CH (OH) CH 2 OH (butylene glycol), 1,4-butanediol HO (CH 2 ) 4 OH (tetramethylene glycol), 2, 3-butanediol CH 3 CH (OH) CH (OH) CH 3 and a tetrol , the butantetrolo HOCH 2 (CHOH) 2 CH 2 OH (eritrolo).
Dall ‘ ethylene CH 2 = CH 2 derives the’ Ethenol CH 2 = CHOH (vinyl alcohol) and propene from CH 2 = CHCH 3 the propenolo CH 2 = CHCH 2 OH (allyl alcohol).
Aldehydes and ketones
They can be defined as compounds deriving from hydrocarbons by formal replacement of two methyl ( aldehydes ) or methylene ( ketones ) hydrogen atoms with an oxygen atom. Their names are derived from those of hydrocarbons with endings respectively ale , -diale , etc. and -one , -dione , etc.
From methane CH 4 (or HCH 3 ) derives only one aldehyde, the methanal HCHO (formaldehyde).
Dall ‘ ethane results in two aldehydes, l’ ethanal CH 3 CHO (acetaldehyde) and l ‘ etandiale CHOCHO (glyoxal).
Two aldehydes derive from propane CH 3 CH 2 CH 3 , propanal CH 3 CH 2 CHO , propandial CHOCH 2 CHO (malonaldehyde) and a ketone, propanone CH 3 COCH 3 (acetone).
From butane CH 3 (CH 2 ) 2 CH 3 derive butanal CH 3 (CH 2 ) 2 CHO , butandial CHO (CH 2 ) 2 CHO , butanone CH 3 COCH 2 CH 3 and butanedione CH 3 COCOCH 3 (diacetyl ).
The propenal CH 2 = CHCHO (acrolein) derives from propene CH 2 = CHCH 3 .
They can be considered as compounds deriving from hydrocarbons by the formal replacement of three hydrogen atoms with an oxygen atom and a hydroxyl -OH. Their names are derived from those of the hydrocarbon with the ending oic acid , -dioico , -trioico , etc.
From methane CH 4 (or HCH 3 ) derives only one acid, the methanoic acid HCOOH (formic acid).
From ethane CH 3 CH 3 derive etanoic acid CH 3 COOH (acetic acid) and etanedioic acid HOOCCOOH (oxalic acid).
Since propane CH 3 CH 2 CH 3 derive the acid propanoic CH 3 CH 2 COOH (propionic acid) and the acid propanedioic HOOCCH 2 COOH (malonic acid).
From butane CH 3 (CH 2 ) 2 CH 3 derive butanoic acid CH 3 (CH 2 ) 2 COOH (butyric acid) and butanean acid HOOC (CH 2 ) 2 COOH (succinic acid).
From 2-methylpropane (CH 3 ) 2 CHCH 3 derives 2-methylpropanoic acid (CH 3 ) 2 CHCOOH (isobutyric acid).
From propene CH 2 = CHCH 3 derives propenoic acid CH 2 = CHCOOH (acrylic acid).
From 2-butene CH 3 CH = CHCH 3 derive 2-butenoic acid CH 3 CH = CHCOOH (crotonic acid, trans ) and butendic acids HOOCCH = CHCOOH (maleic acid, cis ; fumaric acid, trans ).