Fundamentals 1: Molecular identity and properties Flashcards
what is an eclipsed Newman projection?
when the substituents are hiding behind each other.
why is eclipsed less energetically favourable than staggered?
eclipsed atoms = closer together = steric hindrance.
hindrance = repulsion, caused by electron clouds overlapping.
overlapping = higher energy potential for molecule.
electrons are forced to occupy regions of higher energy due to repulsion.
what is a structural isomer?
molecules with the same molecular formula but different spatial arrangements.
what is electronegativity?
the ability of an atom to pull bonding electrons in a covalent bond towards itself.
how much any atom wants electrons, doesn’t want to share.
what is polarisation?
occurs in covalent bonds when one atom attracts the bonding electrons more strongly than the other, due to having different electronegativity values.
what is the inductive effect?
the shifting of electrons in a sigma bond in response to the electronegativity of nearby atoms.
what is a polar molecule?
those which have an overall molecular dipole as a result of polarised bonds within their structure.
what are double bond equivalents?
they describe the degree of unsaturation in a molecule based on its molecular formula.
each DBE is equivalent to one double bond or ring.
are polarised molecules more or less energetically favourable?
polarised molecules = less favourable.
polarisation requires energy to occur, separation of charge creates an electric field that interacts with other charged particles in the environment.
water is one exception, polarisation allows for the forming of hydrogen bonds.
what is a sigma bond?
a type of covalent bond where two atoms share pairs of electrons.
formed by end-on overlap of two atomic orbitals.
electron density is concentrated along the bond axis.
the orbitals can be s, p, or hybrid.
what is transmittance in IR spectroscopy?
the fraction of incident radiation that passes through and reaches the detector.
a measure of the ability of a sample to absorb IR radiation at different frequencies.
used to create an IR spectrum that helps to identify functional groups etc.
why do NMR active nuclei have nuclear spin?
they have an odd number of protons or neutrons.
spin arises from angular momentum of protons/neutrons in the nucleus which = magnetic moment.
as they spin, a magnetic field is induced, they can be aligned with or opposed to an external magnetic field.
with regards to chemical shifts, why are nuclei deshielded when adjacent to electronegative atoms?
electronegative atoms pull electron density away from the nucleus.
creates a local magnetic field that opposes the external magnetic field applied in NMR.
the more electronegative, the more electron-withdrawing effect, and so a greater deshielding.
