Chapter 4 Flashcards
Alkanes
Define saturated hydrocarbon
Hydrocarbons that contain only single bonds between carbon atoms.
- Simplest form of hydrocarbons
- Saturated bc each carbon atom is bonded to as many hydrogen atoms as possible
Define alkane
saturated hydrogen carbons with sigma (single bonds)
- can be in a chain or ring
What are the two types of alkanes?
Acyclic alkanes and cycloalkanes
Define acyclic alkanes
Formula= CnH2n+2
Linear and branched chains
- saturated hydrocarbon bc max number of hydrogens
Define cycloalkanes
Formula = CnH2n
- Makes a ring
- have 2 fewer Hydrogens than acyclic
What is an alkane’s hybridization?
Sp3 and tetrahedral geometry
Define isomers
Two different compounds with the same molecular formula
What are the two main types of isomers?
Constitutional isomers and stereoisomers
Define constitutional isomers
molecules that differ in the way the atoms are connected to each other
Define alkyl group
Formed by removing one hydrogen from an alkane
Are alkanes polar or nonpolar?
Nonpolar
- they only have c-c and c-h bonds
Do alkanes have lower or higher mp’s and bp’s than more polar compounds?
Lower mp and bp bc of less intermolecular forces
How does the mp and bp in alkanes increase?
Increasing the number of carbons increases surface area which increases mp and bp
Does branching on isomers increase or decrease bp?
Branching decreases bp bc less surface area
Does cymmetry increase or decrease mp?
Symmetry increases mp
Define stereochemistry
The 3D structure of molecules
Where does rotation occur in organic molecules?
Around c-c sigma bonds
Define conformations
Different arrangements of atoms that are involved by rotation about single bonds
What are the two types of conformations?
Eclipsed and staggered conformations
Define eclipsed conformations
C-H bond on one carbon is directly aligned with the C-H bond of the adjacent carbon
Define staggered conformations
The C-H bond of one carbon bisects the H-C-H bond angle on the adjacent carbon
Define dihedral angle
The angle that separates a bond on one atom from a bond on an adjacent atom
Does a stable molecule have higher or lower energy?
Lower energy, it’s not excited
Are staggered or eclipsed conformations more stable?
Staggered conformations are more stable (lower in energy)
Define torsional energy
The difference between staggered and eclipsed conformations
Define torsional strain
An increase in the energy caused by eclipsing interactions
Define anti rotations
Staggered conformation with two larger groups 180 degrees from each other
Define gauche rotations
Staggered conformation with two larger groups 60 degrees from each other
Define steric strain
An increase in energy resulting when atoms are forced too close to one another
Are gauche or anti conformations more stable?
Anti conformations are more stable (lower in energy) bc less steric strain
Define angle strain
An increase in energy when tetrahedral bond angles deviate from the optimum angle of 109.5 degrees
What is the most common ring size in naturally occurring compounds?
Cyclohexanes
- 6 carbons
What is the most stable form of a cyclohexane?
The chair form
- all C-C bonds are 109.5 (ideal)
- H’s are staggered and not eclipsed
What is the structure of a cyclohexane?
- 3 up carbons and 3 down carbons
- axial H’s (go up and down)
- equatorial H’s (located in the plane of the ring)
What is a conformation of the chair cyclohexane?
The boat cyclohexane
- the down carbon on the end flips up
How many chair conformations of cyclohexanes exist?
2 with same stability
- up carbons -> down
- axial H’s -> equatorial
Are chair or boat cyclohexanes more stable?
Chair bc more staggered and less torsional strain
Is there more room for large groups on axial or equatorial positions?
Equatorial bs less steric interactions
Define stereoisomers
Isomers that differ only in the way the atoms are oriented in space
What are the two types of stereoisomers?
Cis - two groups on same side of ring
trans - two groups on opposite sides of the ring
In monosubstituted cyclohexanes, is axial or equatorial more favored?
Equatorial
In 1,2-disubstituted cyclohexanes is cis or trans more stable?
Trans bc 2 equatorial sites
In 1,3-disubstituted cyclohexanes is cis or trans more stable?
Cis bc 2 equatorial sites
In 1,4-disubstituted cyclohexanes is cis or trans more stable?
Trans bc 2 equatorial sites
