A reducing sugar is any sugar that has an aldehyde group, or can form one.
key thing 1
– is that an aldehyde group is needed, which must be present on either the 1st or the last carbon.
For sugars like maltose, glucose or lactose to be reducing sugars when they don’t appear to have an aldehyde group, the next point to note is
key thing 2
– the ring structure can be broken to reform an aldehyde on the 1st carbon just like the reversible reaction shown here for glucose.
After that, look closely at Maltose and Lactose to examine if the 1st carbon atom can split with the oxygen in the ring structure to form back an aldehyde on itself just as glucose could in the picture above.
For Maltose, the 1st carbon for the glucose residue on the right is free to reform into an aldehyde! Check that it is the same for Lactose below.
And last but not least, examine this picture of sucrose
Note that to form an aldehyde, the OH on the 1st carbon must give its H to the O in the ring… but there is no OH group on any of the 1st carbons in sucrose!