Chirality in Glucose

why does glucose (when changing from linear to cyclic) form both alpha and beta.
please can someone explain the MECHANISM (I can see the chiral group but cannot understand why the OH sometimes appears up and sometimes down

http://biowiki.ucdavis.edu/Wikitexts/BIS_105%3A_Murphy/Carbohydrates

Okay so just glancing at it:

If you look at the cyclisation step in the diagram above you see the nucleophillic addition of the oxygen (from the alcohol) to the carbonyl carbon to form a tetrahedral intermediate which is protonated to form the alcohol.

Which side of the ring the OH group ends aka whether the glucose ends up either alpha or beta is determined by the approach of the nucleophile, as the OH group will end up anti (opposite) relative to the approach. Since there are no steric factors that would hinder the approach this oxygen can approach from the bottom of the plane and push the OH up. Or it can attack from the top of the plane and push the OH down. Since there are no advantages either way, this is why you will obtain a mixture.

Hope this helped.

thanks so much

why does glucose (when changing from linear to cyclic) form both alpha and beta.
please can someone explain the MECHANISM (I can see the chiral group but cannot understand why the OH sometimes appears up and sometimes down

http://biowiki.ucdavis.edu/Wikitexts/BIS_105%3A_Murphy/Carbohydrates

Okay so just glancing at it:

If you look at the cyclisation step in the diagram above you see the nucleophillic addition of the oxygen (from the alcohol) to the carbonyl carbon to form a tetrahedral intermediate which is protonated to form the alcohol.

Which side of the ring the OH group ends aka whether the glucose ends up either alpha or beta is determined by the approach of the nucleophile, as the OH group will end up anti (opposite) relative to the approach. Since there are no steric factors that would hinder the approach this oxygen can approach from the bottom of the plane and push the OH up. Or it can attack from the top of the plane and push the OH down. Since there are no advantages either way, this is why you will obtain a mixture.

Hope this helped.

Also the OH at the chiral centre is labile due to the anomeric effect. I believe I am right in saying it can be induced to racemise (not actually to an equal distribution as one enantiomer is more stable due to the anomeric effect) with an acid catalyst.