Combine first 5 ingredients in a heavy saucepan; cook over low heat, stirring gently until sugar dissolves. Add pecans; cover and cook over medium heat 2 to 3 minutes to wash down sugar crystals down from side of the pan.
Uncover and cook without stirring to soft ball stage or until candy thermometer reads 235°F.
Remove from heat and stir in baking soda; beat with a wooden spoon just until the mixture starts to thicken (5 to 8 minutes).
Working rapidly, drop by tablespoonfuls onto greased wax paper; let stand until firm.