I'm assuming this question pertains to artificial sweeteners, popular examples being sucralose (in Equal and Splenda) and aspartame (in Nutrasweet).

Sucralose is an indigestible compound formed from sucrose by replacing the OH groups on sucrose sugar molecules with chlorine atoms. There are various ways to perform this synthesis, but in general it is achieved by protecting an OH group that should be retained in the final product, chlorinating to remove the desired groups, then hydrolysis to remove the protective substitute.

Aspartame is a combination of two amino acids. Many synthesis steps are proprietary, but some aspects are known . Nutrasweet is the result of fermentation and synthesis. First, bacteria which produce the appropriate amino acids are cultivated in large quantities. The bacteria are allowed to produce the amino acids until the manufacturer decides a sufficient quantity is present. The amino acids are then separated and dried. One of the amino acids, phenylalanine, is then reacted with methanol to produce and ester. The other amino acid, aspartic acid, is then modified (proprietary step) so it only reacts in the intended manner. The modified acids are then combined at elevated temperatures so they can react. After cooling and adding a solvent, the products of the reaction crystallize. This compound is further reacted with acetic acid. The final aspartame product is filtered to remove unwanted substances.

There are, of course natural sweeteners as well. These are also made through chemical syntheses, but the processes are carried out by living organisms rather than in factories. For example, plants produce sugar (via photosynthesis) and stevia , while bees produce honey .

Depends on the sweetener. Sugar is processed from a grass called sugarcane, which in turn produces the sugar from carbon dioxide in the air and water in the ground using sunlight for energy; this process is called photosynthesis, and also produces oxygen as a waste product. Other sweeteners use different methods, but I am not familiar with them.

There are many different kinds of sweeteners, so it depends on the exact type. But let’s look at sucralose, since it is the most commonly used in America. Sucralose is produced by a multi-step chlorination of sucrose (regular sugar).

If you look at the atomic structure of glucose, you will see it is made up of two six- member rings that look like hexagons, with a lot of OH groups coming off of the corners. To make sucralose, all that happens is three of those OH groups from specific parts of the ring are removed and replaced with Cl atoms. Of course, chemists had to come up with a chemical reaction which does this, and chemical engineers had to come up with a way to make this happen on a large scale. Generally all of the reagents - that is the “ingredients” of a reaction - would be mixed together in a big container called a reactor. The contents would be removed and then purified so that the desired product is isolated. Then, after quality control and safety testing the finished product - the sucralose - can be delivered to people!