A few doubts with Chemistry

intothewild SG

- In tri-iodomethane test, why do we use I2 (aq) alkaline? Can we use Br2(aq) alkaline/heat to achieve the same test for methyl hydroxyl group or methyl carbonyl group?

- For electrophilic substitution reactions of benzene with halogen, the conditions used are X2(g), AlX3 as halogen carrier, and at r.t.p.
Can we use heat for E.S? if so, will it result in further substitution? If it does not, then what other ways can we form di or tri-substituted halo-benze?

- For aqueous iodine, we know that the actual formula is a complex ion of I3-, right? so am I right to say for aqueous bromine, it is also Br3-? if not, then why is it that Br2 can have its aqueous form? I cant seem to figure a reason out of this since both Br and I have d orbitals to expand octet structure.

Thanks!!

UltimaOnline SG

Yes, it works too if bromine is used, but the iodine is preferred as triiodomethane product is less soluble (and is hence a more easily detected precipitate) than tribromomethane.

Heating will increase the rate of reaction and is routinely carried out for most electrophilic aromatic substitutions; but because room temperature will suffice for halogenation in the presence of a Lewis acid catalyst, you’re required to state “room temperature” in the exams to demonstrate that you’re aware room temperature is indeed sufficient (unlike other reactions in which heat is mandatorily required).

And as far as ‘A’ levels (and equivalent, eg. International Baccalaurate) are concerned, heating won’t result in di or tri-substitution of the benzene ring. Notice that each halogen added on further deactivates the benzene ring (as halogens withdraw electrons inductively more significantly than donate electrons by resonance). If di or tri-substitution is desired, you will need to first place an activator (a substitutent that donates electrons by resonance) onto the benzene ring.

Molecular bromine, like all molecular halogens, are only slightly soluble in water. Indeed, so-called “aqueous bromine”, similarly to “aqueous iodine”, are not actually molecular, but exist in complexed forms, most notably X3- (eg. Br3- and I3-). But because the complexed and the molecular forms exist in equilibrium, it’s fine (for ‘A’ levels and equivalent) to write as “Br2(aq)” or “I2(aq)”. Think of the complexed form, eg. I3-(aq), as a “temporary delivery agent” for the I2 molecule in aqueous conditions.

intothewild SG

thanks!!

another question of crystal field splitting.

CFS is for transition element compounds only right?

For some non-transition element compounds, they can also exhibit colors, is it due to the same principle or some other theories involved? I did a google search and read about this" electron-hole center" being " hole color centre"!!! Totally unfamiliar and lengthy...I am just wondering that's all. Thanks again.

UltimaOnline SG

Yes, the nature of chromophores (the structure responsible for colour) arise primarily due to metal complexes (crystal field splitting of d orbitals) or conjugated pi systems (for organic compounds with alternating single and multiple bonds).

The latter is an example of how phenolphthalein and many pH indicators work, and does not involve metal complexes.

intothewild SG

Thanks. another question.

For TM complexes, what determines the no. of ligands that can be formed from a TM ion?
Does it work like if the there are 3d(2)4s2 config for the ion, then the TM ion can accomdate another 8 electrons in the 3d orbitals and therefore another 4 monodentate ligands?

I do not think so right? because most TM ions have so much more 3d orbitals like Cu2+ with 3d9, how can it form a complext with 6 ligands?

Very strange.

UltimaOnline SG

Correct. There are actually many factors involved in determining number of ligands, geometry of the complex ions, and stability of the complx ions, many of which are beyond the requirements or understanding of the ‘A’ level syllabus. The chemistry of complex ions, can be… complex.

You have availability of energetically accessible low-lying vacant orbitals of the transition metal ions to consider. You have thermodynamic and redox stability to consider, including enthalpy and entropy. You have size (sterics) of ligands to consider. You have (‘invisible&#8217 Wink water ligands to consider. You have co-ordination bond lengths and electron geometries to consider. You have crystal field splitting theory to consider. You have ligand exchange/substitution/displacement/replacement to consider. You have Lewis-base strength and crystal field splitting strength of the ligands to consider. Etc etc.

For instance, on the matter of ‘invisible’ water ligands, the deep blue tetraaminecopper(II) ions are actually tetraaminediaquocopper(II) ions, hence making their (complex)ionic geometry square planar rather than tetrahedral. In contrast, other complex ions with ammonia ligands are linear (eg. diammine silver(I) ions) or octahedral (eg. hexaaminenickel(II) ions) with no water ligands.

In some cases, there is ambiguity and even disagreement among chemists on the number of ligands or their geometry, for some complex ions. The Cambridge Mark Scheme should be fair (but this does differ from year to year, depending on the question and mark scheme authors), in that for instance, if only 4 ligands are indicated for a relatively obscure complex ion, both “tetrahedral” and “square planar” may be acceptable.

For ‘A’ levels, it’s more practical and convenient to simply be familiar (ie. memorize) a list of common transition metal complex ions and their geometries, rather than go into greater depth on the subject than is required by the ‘A’ level syllabus.

For this topic on Complex Ions :

1) Jim Clark’s website is excellent and thoroughly suffices for ‘A’ levels. http://www.chemguide.co.uk/inorganic/complexmen…

2) For users of CS Toh’s ‘A’ Level Study Guide, his ‘Advanced’ Study Guide (a thicker version covering multiple Chemistry syllabuses including H2 Singapore ‘A’ levels, International Baccalaurate Chemistry, and other non-local ‘A’ level syllabuses) is actually more recommended, specifically or only for this topic on Complex Ions. (For all other topics, the original ‘A’ Level Study Guide will suffice).

dkcx SG

For this topic on Complex Ions :

1) Jim Clark’s website is excellent and thoroughly suffices for ‘A’ levels. http://www.chemguide.co.uk/inorganic/complexmen…

Isn't there specific transition metals that will specifically choose a square planar or tetrahedral arrangement? Not sure about A's but in uni we are wrong if we put a known geometry compound as the other.