Large substituents can interfere with rotations and slightly alter bond angles {steric hindrance}|.
Organic molecules have three-dimensional arrangements {conformation, molecule}|. Carbon-chain substituents can have alternate forms {carbon chain conformation}.
Carbon-chain substituents can align {eclipsed form}. Eclipsed form has more hindrance, more potential energy, and less probability.
Carbon-chain substituents can alternate position {staggered form}. Staggered form has less hindrance, lower potential energy, and more probability.
The two highest-priority groups can be on same double-bond side {cis, orientation}|, designated by prefix Z.
The two highest-priority groups can be on opposite double-bond sides {trans, orientation}|, designated by prefix E.
In six-carbon rings with only single bonds, carbons have single bonds to two substituents. Whole ring {ring configuration} can look like a chair {chair configuration} or boat {boat configuration}.
If adjacent carbons have one substituent and one hydrogen atom, substituents can both be axial or equatorial {trans, ring} or can be one axial and one equatorial {cis, ring}. Single bonds between carbon atoms allow rotation. Because ring bond rotation must change two single bonds simultaneously, bond rotation cannot change cis to trans, or trans to cis.
Bonds {axial bond} to substituents can point perpendicular to rings.
Bonds {equatorial bond} to substituents can point parallel to rings. Large substituents tend to be equatorial.
Molecules {isomer}| with same atoms can have different chemical and physical arrangements. Isomers {structural isomer} can have different chemical bonds among chain atoms. Isomers {functional isomer} can have different chemical bonds among substituents.
To distinguish and name isomer, give atom with highest atomic number highest priority. For atoms with same atomic number, use atomic mass to assign priority. If two or more atoms are the same, go outward from chiral atom, applying same rules to farther atoms. Treat double bonds like two single bonds with atoms.
Isomers {conformational isomer} can have same chemical bonds, but rotations around single bonds make different spatial relations.
Isomers {diastereomer}| that are not mirror images can be unable to superimpose.
Double bonds between two carbon atoms make all bonds lie on one plane. The four single bonds can have different isomers {geometric isomer}. The two highest-priority groups can be on opposite double-bond sides {trans, double bond}, with prefix E. The two highest-priority groups can be on same double-bond side {cis, double bond}, with prefix Z.
Structural isomers can transfer proton between two non-equivalent sites {tautomerism}|. Aldol and ketol exhibit tautomerism.
Isomers {stereoisomer}| {optical isomer} can have right-handed and left-handed structures that single-bond four different substituents to carbon atom. Stereoisomers are asymmetric molecules, such as enantiomer, racemic, diastereomer, and meso compound. The D and L system {glyceraldehyde} designates stereoisomers relative to (+)-glyceraldehyde {D-glyceraldehyde} and (-)-glyceraldehyde {L-glyceraldehyde}.
For R-S systems {Cahn-Ingold-Prelog system} {CIP system}, at asymmetric carbon atom, place lowest-atomic-number bound atom straight-behind the plane, with the three other atoms facing observer. Mark highest-atomic-number atom and next highest. For atoms with same atomic number, use atomic mass. If two or more atoms are the same, go outward from chiral atom, applying same rules to farther atoms. Double bonds, triple bonds, and aromatic rings are like two or three bonds with atoms. If highest to next highest goes clockwise, compound is R or Rectus. If highest to next highest goes counterclockwise, compound is S or Sinister.
Isomers {meso compound}| have asymmetric carbon but also have symmetry plane that cancels optical rotation.
Two isomers {enantiomer}| {enantiomorph} can be mirror images.
Mixtures {racemic mixture} can have equal enantiomer amounts.
Reactions {racemization}| can change one enantiomer into the other.
Stereoisomers can rotate plane-polarized light clockwise {dextrorotatory} (+), as measured by degrees in polarimeter.
Stereoisomers can rotate plane-polarized light counterclockwise {levorotatory} (-), as measured by degrees in polarimeter.
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Date Modified: 2022.0225