How to deduce the general formula of alkanes (according to the ratio of hydrocarbon atoms such as methyl and ethyl) Why methane is still alkanes after all four hydrogens are replaced by methyl (mathematical analysis) First, "alkane" and "alkane" are two different concepts! If so, you should mean "alkanes".

Methane is 1C4H. All alkanes are evolved from methane.

The definition of alkane is that every carbon-carbon bond in the molecule is a single bond (saturated).

So what do you find when the substituent replaces methane (-CH3 replaces -H)? One more C, two more H, pay attention to two! That's very simple. When alkyl is substituted, it is equivalent to adding methylene (-CH2-) one at a time. Let C(B) represent the number of carbon in b and H(B) represent the number of hydrogen in b, then

C(CH4)= 1，H(CH4)=4，H(CH4)=2*C(CH4)+2

Adding one more methylene at a time is equivalent to 2*C(CH4). Add C (CH2) = 1 and H (CH2) = 2 here, and that "+2" will never change! Therefore, the general formula of alkanes is C2nH2n+2.

There is also a deductive method. Alkanes can be regarded as a string (H-CH2-CH2-CH2-H), and each isomer is just the transposition of H and a certain CH3 in the molecule. This is easy to understand: every alkane has CH2 in the middle and two H at the end. So the problem is very clear, and the general formula is C2nH2n+2.