The oxygen in the rings and the oxygen between the rings of sucrose are actually two acetal groups. When sucrose is dissolved in diluted acid the oxygen between the two rings gets hydrolyzed and the rings separate. This in effect causes the glucose and fructose rings to open up into their linear structure. In the linear form, sulfuric acid removes the OH groups from each carbon. The aldehyde group of C1 in each individual sugar gets reduced to an alcohol and then gets removed like the other alcohols.The second hydrogen of sulfuric acid ionizes partially so it probably does contribute to the dehydration of the sucrose. Yes, the reaction does get hot enough for SO2 to form.
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