A 27 1/2" tube is not really the same as 700c. 622/700c is a pretty common size that has a bead seat diameter of 622mm. 27 1/2" is a rare Danish size with a bead seat diameter of 609mm. Tubes fit a range of sizes for a specific diameter tire; the 26" tube might fit the 26" tire, but you need a 700 x whatever width range covers your tire width for the 700c wheel. For example, Conti makes a 700x32-47c that fits a 700x45c tire.
Further complicating things, fractional tire/tubes are not the same as mathematically equivalent decimal sizes: 26x1.75" is not the same as 26x1 3/4".
If you want a really good explanation, check out Sheldon Brown.
A 27 1/2" tube is not really the same as 700c. 622/700c is a pretty common size that has a bead seat diameter of 622mm. 27 1/2" is a rare Danish size with a bead seat diameter of 609mm. Tubes fit a range of sizes for a specific diameter tire; the 26" tube might fit the 26" tire, but you need a 700 x whatever width range covers your tire width for the 700c wheel. For example, Conti makes a 700x32-47c that fits a 700x45c tire.
Further complicating things, fractional tire/tubes are not the same as mathematically equivalent decimal sizes: 26x1.75" is not the same as 26x1 3/4".
If you want a really good explanation, check out Sheldon Brown.