Benzyl chloride and butyl halide each react with CO2 catalyzed by 5,10,15,20-tetraphenylporphyrin cobalt (CoTPP) to form phenylacetic acid (in the form of benzyl phenylacetate, the product of electrocarboxylation) or oxalic acid (the direct reduction product of CO2, in the form of dibutyl oxalate), respectively. The data show that electrocarboxylation of benzyl chloride reacting with CO2 can occur, but butyl halide in the presence of CO2 cannot produce the electrocarboxylation product amylic acid. The reasons for the different mechanisms are examined through the reaction of alkyl halide with CoTPP studied by electrochemistry and spectroelectrochemistry. The intermediate N-alkyl CoTPP is detected by spectroelectrochemistry in situ. The reasons that they form different products with different N-alkyl CoTPP are discussed in detail.