CHEM 1020 MIDTERM I EXAM REVIEW QUESTIONS
WITH COMPLETE SOLUTIONS
What is the formula for an acyclic alkane? -- Answer ✔✔ C(n)H(2n+2)
How is an alkyl group formed? -- Answer ✔✔ by removing one hydrogen from the chain
of an alkane
What are the IUPAC rules for naming alkanes? -- Answer ✔✔ 1) find and name the
longest continuous chain
2) identify and name the groups attached to the chain
3) number the chain consecutively, starting at the end nearest to a substituent group
4) designate the location of each substituent group by an appropriate number and name
5) assemble the name, listing the groups in alphabetical order
6) the prefixes di-, tri-, and tetra- are used to designate several groups of the same kind,
but are not considered when alphabetizing
How to name alkyl halides -- Answer ✔✔ halogen substitutes are inserted alphabetically
into the name the same way that alkyl groups are included when naming alkanes
,How are ethers named? -- Answer ✔✔ name each of the carbon groups as a separate
word, followed by a space and the word "ether"; the smaller and shorter alkyl group
becomes the alkoxy substituent, and the larger and longer alkyl group becomes the
alkene base name
What are structural isomers? -- Answer ✔✔ alkanes with four or more carbon atoms
can have more than one arrangement of atoms (i.e. a single unbranched chain vs a
primary chain with one or more branches)
What state do the first four alkanes take at room temperature? -- Answer ✔✔ gases
How do the boiling points of cycloalkanes differ from straight-chain alkanes? -- Answer
✔✔ they are about 10-20 degrees higher than their corresponding straight chain alkane
Why do boiling points of alkanes increase with size? -- Answer ✔✔ because the van der
Waals forces are very small for smaller alkanes, but increase as the size of the alkane
increases
Why do branched chain alkanes have lower boiling points? -- Answer ✔✔ because the
van der Waals forces are smaller for shorter molecules, and only operate over very
short distances between one molecule and its neighbors
What is the solubility of alkanes? -- Answer ✔✔ insoluble in water but dissolvable in
organic solvents
Why are alkanes insoluble in water? -- Answer ✔✔ the only new attractions between an
alkane and water are the van der Waals forces, which do not release a sufficient amount
of energy to compensate for the energy required to break the hydrogen bonds in water
, Why are alkanes soluble in organic solvents? -- Answer ✔✔ because the primary forces
of attraction between the solvent molecules are the van der Waals forces (either
dispersion or dipole-dipole attractions), so when an alkane dissolves, the van der Waals
forces are broken and replaced by new ones, and these two processes cancel out
energetically, so there is no barrier to solubility
Why aren't alkanes very reactive? -- Answer ✔✔ because they contain strong C-C and
C-H bonds, and there are no portions of an alkane molecule that carry any significant
amount of charge, which is required for other molecules to be attracted
Why are very small cycloalkanes more reactive than expected? -- Answer ✔✔ because
the bond angles in the ring cause a significant amount of repulsion between the bonding
pairs joining the carbon atoms, making the bonds easier to break
What is a dihedral angle? -- Answer ✔✔ the angle that separates a bond on one atom
from a bond on an adjacent atom
What is the name for two atoms with a dihedral angle of 0 degrees? -- Answer ✔✔ syn-
periplanar
What is the name for two atoms with a dihedral angle of 180 degrees? -- Answer ✔✔
anti-periplanar
What is torsional strain? -- Answer ✔✔ the twisting force that a molecule experiences
when a C-C bond undergoes rotation from a staggered to an eclipsed conformation
WITH COMPLETE SOLUTIONS
What is the formula for an acyclic alkane? -- Answer ✔✔ C(n)H(2n+2)
How is an alkyl group formed? -- Answer ✔✔ by removing one hydrogen from the chain
of an alkane
What are the IUPAC rules for naming alkanes? -- Answer ✔✔ 1) find and name the
longest continuous chain
2) identify and name the groups attached to the chain
3) number the chain consecutively, starting at the end nearest to a substituent group
4) designate the location of each substituent group by an appropriate number and name
5) assemble the name, listing the groups in alphabetical order
6) the prefixes di-, tri-, and tetra- are used to designate several groups of the same kind,
but are not considered when alphabetizing
How to name alkyl halides -- Answer ✔✔ halogen substitutes are inserted alphabetically
into the name the same way that alkyl groups are included when naming alkanes
,How are ethers named? -- Answer ✔✔ name each of the carbon groups as a separate
word, followed by a space and the word "ether"; the smaller and shorter alkyl group
becomes the alkoxy substituent, and the larger and longer alkyl group becomes the
alkene base name
What are structural isomers? -- Answer ✔✔ alkanes with four or more carbon atoms
can have more than one arrangement of atoms (i.e. a single unbranched chain vs a
primary chain with one or more branches)
What state do the first four alkanes take at room temperature? -- Answer ✔✔ gases
How do the boiling points of cycloalkanes differ from straight-chain alkanes? -- Answer
✔✔ they are about 10-20 degrees higher than their corresponding straight chain alkane
Why do boiling points of alkanes increase with size? -- Answer ✔✔ because the van der
Waals forces are very small for smaller alkanes, but increase as the size of the alkane
increases
Why do branched chain alkanes have lower boiling points? -- Answer ✔✔ because the
van der Waals forces are smaller for shorter molecules, and only operate over very
short distances between one molecule and its neighbors
What is the solubility of alkanes? -- Answer ✔✔ insoluble in water but dissolvable in
organic solvents
Why are alkanes insoluble in water? -- Answer ✔✔ the only new attractions between an
alkane and water are the van der Waals forces, which do not release a sufficient amount
of energy to compensate for the energy required to break the hydrogen bonds in water
, Why are alkanes soluble in organic solvents? -- Answer ✔✔ because the primary forces
of attraction between the solvent molecules are the van der Waals forces (either
dispersion or dipole-dipole attractions), so when an alkane dissolves, the van der Waals
forces are broken and replaced by new ones, and these two processes cancel out
energetically, so there is no barrier to solubility
Why aren't alkanes very reactive? -- Answer ✔✔ because they contain strong C-C and
C-H bonds, and there are no portions of an alkane molecule that carry any significant
amount of charge, which is required for other molecules to be attracted
Why are very small cycloalkanes more reactive than expected? -- Answer ✔✔ because
the bond angles in the ring cause a significant amount of repulsion between the bonding
pairs joining the carbon atoms, making the bonds easier to break
What is a dihedral angle? -- Answer ✔✔ the angle that separates a bond on one atom
from a bond on an adjacent atom
What is the name for two atoms with a dihedral angle of 0 degrees? -- Answer ✔✔ syn-
periplanar
What is the name for two atoms with a dihedral angle of 180 degrees? -- Answer ✔✔
anti-periplanar
What is torsional strain? -- Answer ✔✔ the twisting force that a molecule experiences
when a C-C bond undergoes rotation from a staggered to an eclipsed conformation
