[ABC]

In 3-chloro-1-pentene, there are 4 groups attached to carbon: Cl, H2C=CH, CH2CH3, and H. My textbook orders Cl first, H2C=CH second, CH2CH3 third, and H last. I can understand the reasons for Cl and H, but how do you prioritize H2C=CH and CH2CH3? They both move from C to H. Is H2C=CH of 2nd priority because it has a double bond?

[UG]

If you cannot make a decision based on the first atom, look at the second, third or fourth atom etc... until the first difference is found. One of the CIP rules is that multiple bonds are equivalent to the same number of single bonded atoms. With your example, the H₂C=CH group is equivalent to H₂C(C)-C(C)H (The C atoms in brackets are bonded to the unbracketed carbons). Now compare this with the CH₂CH₃ group. Looking at the first atom away from the chiral center atom, both groups have a C atom. Looking at the second atom away from the chiral centre, both groups have a C atom. But looking at the third atom away from the chiral centre, H₂C(C)-C(C)H has a carbon atom whereas CH₂CH₃ has a hydrogen atom, so the one with carbon as the third atom gets higher priority.

[ABC]

If you cannot make a decision based on the first atom, look at the second, third or fourth atom etc... until the first difference is found. One of the CIP rules is that multiple bonds are equivalent to the same number of single bonded atoms. With your example, the H₂C=CH group is equivalent to H₂C(C)-C(C)H (The C atoms in brackets are bonded to the unbracketed carbons). Now compare this with the CH₂CH₃ group. Looking at the first atom away from the chiral center atom, both groups have a C atom. Looking at the second atom away from the chiral centre, both groups have a C atom. But looking at the third atom away from the chiral centre, H₂C(C)-C(C)H has a carbon atom whereas CH₂CH₃ has a hydrogen atom, so the one with carbon as the third atom gets higher priority.

Hm... I am still not sure. When you said that multiple bonds are equivalent to single bonded atoms, do you mean having the double bonds is no different than single bonds? or do you mean that the double bonds mean carbon? Where does the bracketed C come from?

[UG]

Apologies if I wasn't very clear. Perhaps a visual explanation is better suited.
See if this clears things up.