A bit of trouble I am having with grasping the concept of fischer projections for long chained molecules. Take any non-cyclic carbohydrate from a tetrose and upwards and make an actual model of it and what we see is that the carbon backbone follows a zigzag pattern. However, according to the fischer projections, if we take any chiral carbon the vertical bonds go into the plane of the paper and the horizontal out of the plane of the paper. With a real-life model which follows a tetrahedral structure at each carbon atom and a zigzag pattern this just doesn't seem to work. What is going on here??
The carbon chain of a fischer projection is drawn vertically.
Yes, the vertical and horizontal lines depict 'into the page' and 'out of the page' respectively.
However, when you have a carbon chain with more than 3 carbons, each down carbon on the backbone which would normally be projected away from you (according to the above rule) has to be rotated. Essentially, this creates the 'zig-zag' you would see in other projections (e.g. Natta Projection).